When Guy Burgess's flat was searched in 1951, security men found a bundle of handwritten notes confirming affairs inside the Treasury. There were also pen portraits of various officials written as though by a talent scout giving information about character weaknesses and other features that might be exploited. The notes dated from the early 1940s, and their continued existence was evidence of the carelessness and slovenliness in Burgess's character, so atypical of a highly successful spy, as he undoubtedly was. The papers were not signed, and their author might never have been traced but for a fluke occurrence. An MI5 case officer, who had acquired a new secretary from Whitehall, sent for the file in which the papers happened to have been placed. She recognized the handwriting as that of her previous superior, a young civil servant called John Cairncross, who had been on the Treasury staff in Whitehall in 1940.

As it already seemed certain that Burgess had been a Soviet recruiter and active spy, it seemed likely that the Treasury information, which could have been of value to the Russians, had been provided as an espionage service.
It was known that Cairncross had been a scholarship boy from a poorly off home in Glasgow who had gone to Cambridge, where he had done brilliantly in modern languages. It was soon discovered that he had been an overt Communist in 1935 when, though intending to pursue an academic career, he had suddenly changed course to enter the Foreign Office.

When confronted with the notes early in 1952 by MI5's Arthur Martin, Cairncross denied being a spy or any kind of Soviet agent. He admitted having supplied the notes at Burgess's request but said that he had no knowledge that Burgess was a Soviet agent and did not believe it could possibly be true.

As Cairncross had to concede that he had written the notes and that they contained some classified information, which could have been of value to a foreign power, and particularly to the Russians in their political negotiations with the Germans in 1940, he offered to resign. His resignation was accepted, and he obtained a post with the UN Food and Agriculture Organization in Rome.

The public heard nothing of this until after Blunt's exposure in 1979, when Cairncross openly acknowledged his former communism and his unfortunate association with Burgess and Blunt. When questioned by journalists in Rome, he admitted that he had given Burgess the offending notes and had resigned as a consequence without a pension but, understandably, volunteered nothing further. He has, therefore, since been dismissed from the haul of known Soviet agents as "small fry."

The truth is very different. As an active spy throughout the war, in highly sensitive positions, Cairncross was a "big fish" and did great damage to his country.

While Blunt always attempted to cover his close friends during his interrogations, he was open about Cairncross, whom he seemed to dislike. He admitted having talent-spotted him as a potential spy while teaching him at Cambridge and having alerted Burgess to this effect, though the actual recruitment had been achieved by an even more sinister Communist agent, James Klugman.

An MI5 officer therefore traveled to Rome to interrogate Cairncross, who, knowing that he was outside the jurisdiction of the British Official Secrets Act, made a complete and contrite confession of his treachery.

He admitted what the MI5 men already knew about his recruitment to the service of the KGB, explaining that he had experienced poverty and had concluded that Soviet-style communism was the only way of securing social justice, though he claimed that he had since realized he had been hopelessly misled in this respect by other Marxists.

He disclosed that Klugman had introduced him to the ubiquitous "Otto" on a special visit to Regent's Park, where they were unlikely to be seen. In accordance with the usual practice, "Otto," who was also running Burgess, Maclean, Philby, and Blunt, had instructed him to reject his open communism, go "underground," and get himself into the Foreign Office instead of pursuing an academic career, as he would have preferred. Cairncross officially quit the Communist party in late 1936.

Cairncross recalled how his Glaswegian accent was of some concern to "Otto," who felt that he needed to improve his diction if he was ever to get into the top echelons of Whitehall. "Otto" also advised him against marrying a "bourgeois" wife because he had already lost a very promising agent, whom he had recruited at Cambridge, through marriage to a woman too "bourgeois" to condone his spying for Russia. (MI5 is confident that it knows the identity of this short-time agent, who is now a life peer.)

Acting on "Otto's" instructions, Cairncross competed for entry to the Foreign Office, passed top of his list, and began work there in the German department, where Donald Maclean was also then located. Cairncross remained in the Foreign Office for two years, and he admitted that after "Otto" had been recalled to Moscow in September 1938, he handed his documentary material to Burgess, who passed it to Litzi Philby, Kim Philby's estranged Austrian wife, who was then working in London as a full-time Soviet agent.

Litzi had been a militant Communist, divorced and living with her parents in Vienna, where Philby had married her in 1934. He brought her to London soon afterward, and so superficial was the checking of entrants into the Secret Service that nobody in authority knew that Philby was married to a Russian spy until 1946, when he needed a divorce. Even then, no effective notice was taken of it.

In 1938, at the suggestion of his Soviet controllers, Cairncross applied for transfer to the Treasury. This is believed to have been preferred by the Russians because Maclean was already covering far them in the Foreign Office.

Cairncross admitted that in his early days he had been given money by the Russians but only in small amounts as expenses. This included the purchase price of a cheap motorcar to facilitate contacts with Soviet controllers outside London.

Philby was slaving for the Soviet cause, risking his life and prepared to report on whatever interested Moscow, including his father, his wife and his best friend.Yet Moscow was not happy. The Cambridge spies - Guy Burgess in MI6, Donald Maclean in the Foreign Office, Anthony Blunt in MI5, John Cairncross in Bletchley Park and Kim Philby in Section V - were producing top-level intelligence. But their very productivity posed a conundrum. In the insanely distrustful world of Soviet espionage, the quality, quantity and consistency of this information rendered it suspect. A misgiving began to take root in Moscow that British intelligence must be mounting an elaborate, multi-layered deception through Philby and his friends; they must all be double agents. Moreover, Philby's story failed to meet firmly held Soviet preconceptions: MI6 was supposed to be impregnable, yet Philby had practically sauntered into the organisation; he had been a left-winger at university, yet supposedly rigorous background checks had failed to pick this up; he had been asked to find evidence that his own father was a spy, and had failed to do so.

I was posted to the secret and vital operation of deciphering signals of foreign military forces carried out at the Government Code & Cipher School (GC&CS), which had moved shortly before the war from the SIS headquarters in London to the small town of Bletchley, some sixty miles on the main railway line to the north, where it was safely hidden from German bombers. The school itself would not have aroused suspicion, set in the grounds of a hideous nineteenth-century Victorian mansion, which would serve as its administrative headquarters. But if one looked through the high steel fence which guarded the compound from intruders, one could see a cluster of dingy prefabricated huts of the type normally found in army camps. It was in these unimpressive structures that the most important technical breakthrough of the Second World War took place: the cracking of the ENIGMA cipher machine, a model of which had been provided to the British by a courageous group of Polish cryptographers who had been working for the Germans. Yet the final breakthrough was due to the skill and tenacity of British experts.

The staff at Bletchley Park, but mainly its cryptographers and other technicians, was a mixed group, chaotically assembled during the early years of the war. They were not the typical product of the older universities, where the cult and cultivation of social polish and homogeneity were mostly prized above scientific and commercial ingenuity. Churchill himself made the point with his usual pungent humour, in a rapid visit to the GC&CS in 1941, when he commented to Commander (Sir) Edward Travis, the Director: "I know I told you to leave no stone unturned to find the necessary staff, but I didn't mean you to take me literally." But these were the experts who produced solutions of genius for the nation's wartime problems by developing an even more ingenious device than the ENIGMA machine itself. This machine, designed by the mathematical genius Alan Turing, was a forerunner of the modern computer and reduced the laborious process of examining the infinite possibilities of interpretation to manageable proportions. The culmination of these efforts was that the unintelligible texts transmitted by the Germans in cipher could be "unscrambled" and restored to their original text. This was not a once and for all battle, but a struggle to meet a constant challenge, since it was possible for the settings of the ENIGMA machine to be endlessly changed by the Germans; but the British technicians rose to the challenge. (We linguists only rarely caught a glimpse of the difficulties encountered by the technical side in the decipherment of the signals, which called for a different sort of mind.)

The linguists were more typical of an Oxbridge educational background and were later recruited in greater numbers in order to process the fruit of the progress made by the technicians. As fluency in German was a very scarce commodity in England, I was automatically assigned to ENIGMA when I was called up. I suppose my position as Private Secretary to Lord Hankey had been a sufficient guarantee of reliability, and I was taken on by the GC&CS and not even subjected to any interrogation.

After a short training in simple (non-machine) codes at nearby Bedford, which never proved useful, I was sent to Bletchley and put to work. We lived a semi-monastic life, which was only broken by the occasional visit to London to recuperate. There was a direct train to London, so that travel back to my flat there on my day off was not a problem. I had no car at the time and indeed not even a driving licence, but GC&CS was within walking distance of the railway station. Weekends were unheard of since the operational work was non-stop. Social functions too were virtually ruled out by working conditions and there were no common rooms. The rigid separation of the different units made contact with other staff members almost impossible, so I never got to know anyone apart from my direct operational colleagues. We did eight fully-occupied hours of work and then were transported back to our respective lodgings with families in the surrounding villages.

Even within my hut, I never met some of the more important personalities, such as Peter Calvocoressi, who wrote a book on his experiences at Bletchley. Except for the work and the routine, I remember very little of what happened there during my twelve months' service. My territory was limited to my hut and to the functional and austere cafeteria, which could hardly be described as having a relaxed and inviting atmosphere.

When I discovered the nature of the work I was to be engaged in, I was proud to take part in this superb achievement of British brains, and was soon fascinated with the job itself. Few of us were military experts or had any knowledge of the details of the fighting, so our satisfaction was with the work itself. I found the editing of the German decrypts much like solving a crossword puzzle, or amending a corrupt text of a classical writer such as Moliere. My work involved the correction and restoration of words blurred, distorted or omitted. This was a task which needed a generous dose of imagination, and a corkscrew mind.

The translators/editors operated in groups of six, including a team leader. The German ENIGMA (ULTRA SECRET) decrypts came in rolls of paper three or four feet long, each roll containing some ten signals. The leader's task was to decide whether the signals were worth processing (as was rarely the case), to check out if translations were accurate, and to make sure that no information was overlooked which had a tactical or organisational significance. For instance, at the beginning of my new career, I overlooked the implications of a particular phrase containing a reference to a German Luftwaffe unit in Yugoslavia, which could have been identified by relating this passage to a previous signal received two days earlier. There was another instance later on in which a passage did not seem to make sense, no matter how hard I racked my brains trying out various solutions. It turned out that two signals had been run together, and that this crucial factor had been overlooked by the expert attached to the cryptographic section.

The team leader had to ascribe a fictitious source for each signal and ensure that it was plausible, for the translation was careful never to give the slightest hint of the real origin of the document. The kind of source ascribed, for example, would be a mythical British agent such as an officer in the German Army High Command (OKW). Our product was known as the "sanitised" version of ULTRA SECRET and was the one supplied to all recipients, including the War Office...

I had been greeted on my first day at Bletchley by the receiving officer who explained to me the billeting system and informed me about other practical matters such as transportation to and from work. He emphasised the utterly secret nature of the decipherment operations and the need for complete secrecy in all our work, since, if the Germans suspected that Britain was reading ENIGMA, they would change the cipher and we would take a long time, if ever, to break into it again. We might even lose the war as a result. He ended the interview with the striking, if casual, announcement that we had not confided our ENIGMA triumph to the Russians "because we do not trust them". They were, he implied, a security risk.

This offhand announcement shook me and set my mind racing. I had arrived at Bletchley with the determination to sever my connection with the KGB. I felt certain that the Government, and Churchill in particular, would not have excluded the Russians from this important source of intelligence without the I had been greeted on my first day at Bletchley by the receiving officer who explained to me the billeting system and informed me about other practical matters such as transportation to and from work. He emphasised the utterly secret nature of the decipherment operations and the need for complete secrecy in all our work, since, if the Germans suspected that Britain was reading ENIGMA, they would change the cipher and we would take a long time, if ever, to break into it again. We might even lose the war as a result. He ended the interview with the striking, if casual, announcement that we had not confided our ENIGMA triumph to the Russians "because we do not trust them". They were, he implied, a security risk.

There were two most probable grounds for the ban. I speculated and feared that even if the Allies won the war, their basic differences would, in the not-too-far-distant future, lead to the parting of ways. Even now relations were far from perfect. I recall Churchill's Private Secretary, John Colville, telling me that his master had once confided in him that there was no limit to the deceptiveness of the Russians. The Molotov-Ribbentrop Pact may have been thrust aside for the moment, just as the long list of Stalin's horrors was tactfully overlooked, but they could not be extinguished or forgotten. The other consideration was a technical but vital one. The Germans might now or later crack the Russian military cipher, and in that case, since the Russians would be making the most of ENIGMA information in their traffic, the Germans would soon be aware that their own cipher had been read.

Cairncross was a different character entirely. He was a clever, rather frail-looking Scotsman with a shock of red hair and a broad accent. He came from a humble working-class background but, possessed of a brilliant intellect, he made his way to Cambridge in the 1930s, becoming an open Communist before dropping out on the instructions of the Russians and applying to join the Foreign Office.

Cairncross was one of Arthur's original suspects in 1951, after papers containing Treasury information were found in Burgess' flat after the defection. Evelyn McBarnet recognized the handwriting as that of John Cairncross. He was placed under continuous surveillance, but although he went to a rendezvous with his controller, the Russian never turned up. When Arthur confronted Cairncross in 1952 he denied being a spy, claiming that he had supplied information to Burgess as a friend, without realizing that he was a spy. Shortly afterward, Cairncross left Britain and did not return until 1967.

The Centre calculated that since their recruitment in 1934-5, Philby, Burgess and Maclean had supplied more than 20,000 pages of 'valuable' classified documents and agent reports. As Philby had feared, however, the defection of Burgess and Maclean did severe, though not quite terminal, damage to the careers in Soviet intelligence of the other members of the Magnificent Five. Immediately after the defection, Blunt went through Burgess's flat, searching for and destroying incriminating documents. He failed, however, to notice a series of unsigned notes describing confidential discussions in Whitehall in 1939. In the course of a lengthy MI5 investigation, Sir John Colville, one of those mentioned in the notes, was able to identify the author as Cairncross. MI5 began surveillance of Cairncross and followed him to a hurriedly arranged meeting with his controller, Modin. Just in time, Modin noticed the surveillance and returned home without meeting Cairncross. At a subsequent interrogation by MI5, Cairncross admitted passing information to the Russians but denied being a spy. Shortly afterwards he received "a large sum of money" at a farewell meeting with Modin, resigned from the Treasury and went to live abroad.

After Cairncross confessed, Arthur and I traveled to Paris to meet him again for a further debriefing in a neutral venue. He had already told Arthur the details of his recruitment by the veteran Communist James Klugman, and the intelligence from GCHQ and MI6 which he had passed to the Russians, and we were anxious to find out if he had any other information which might lead to further spies. Cairncross was an engaging man. Where Long floated with the tide, Communist when it was fashionable, and anxious to save his neck thereafter, Cairncross remained a committed Communist. They were his beliefs, and with characteristic Scottish tenacity, he clung to them. Unlike Long, too, Cairncross tried his best to help. He was anxious to come home, and thought that cooperation was the best way to earn his ticket.

Cairncross said he had no firm evidence against anyone, but was able to identify two senior civil servants who had been fellow Communists with him at Cambridge. One was subsequently required to resign, while the other was denied access to defense-related secrets. We were particularly interested in what Cairncross could tell us about GCHQ, which thus far had apparently escaped the attentions of the Russian intelligence services in a way which made us distinctly suspicious, especially given the far greater numbers of people employed there.

Cairncross told us about four men from GCHQ who he thought might repay further investigation. One of these worked with him in the Air Section of GCHQ, and had talked about the desirability of enabling British SIGINT material to reach the Soviet Union. Cairncross, although amused by the irony of the man's approach, was in no position to judge his seriousness, so he kept quiet about his own role. The second man, according to Cairncross, had been sacked after returning to Oxford and telling his former tutor full details of his work inside GCHQ. The tutor, appalled by the indiscretion, reported him to GCHQ, and he was sacked. A third man named by Cairncross, like the first, had long since left GCHQ for an academic career, so effort was concentrated on the fourth, a senior GCHQ official in the technical section. After a full investigation he was completely cleared.

GCHQ became highly agitated by the D Branch inquiries resulting from Cairncross' information, as did C Branch; both protected their respective empires jealously, and resented what they saw as interference, particularly when I made some caustic comments about how they could improve their vetting.

John Cairncross, the last survivor of the KGB's "Ring of Five", was a testament to misconceived idealism among Britain's intelligentsia in the 1930s and to the futility of MI5's hunt for Britain's Communist traitors.

He was born outside Glasgow in 1913, one of four brothers and four sisters; their father ran an ironmonger's shop while his mother was a primary school teacher. From those inauspicious but radical beginnings, three brothers became professors, including the noted economist Sir Alec Cairncross. Academia would also have welcomed John Cairncross, whose original research and books became internationally renowned.

After leaving Glasgow University in 1933 with a degree in French and German, Cairncross was awarded another degree at the Sorbonne before winning a scholarship to Trinity, Cambridge, where his fluency in languages was less remarked upon than a cantankerous and arrogant manner.

In the political cauldron of that era, Cairncross did not stand out as a political activist or a member of any group although he did join Cambridge's Modern Language Society, an organisation with links to the Communist Party. There, his left-wing sympathies were noted by Anthony Blunt. The KGB's talent-spotter disliked Cairncross as an unsociable, insipid personality, and the sentiment was reciprocated. Cairncross was only approached by the KGB in 1936, after he joined the Foreign Office having topped the entrance exams.

His recruiter was James Klugman, one of Cambridge's most influential Marxists. The approach was classic. Cairncross was invited to help the Comintern, the international Communist movement, against Fascism. His seething hatred of the British establishment was the impetus to treachery. His earlier failure to join the Communist Party was a bonus. In perfect tradecraft, Klugman did not mention to his new recruit the names of others who were helping the Soviet cause. It was also wise, because Cairncross, besides disliking Blunt, had met Donald Maclean in the FO's Western Department and instantly loathed another of the KGB's Cambridge recruits on account of his fellow Scot's charm. Until 1951 Cairncross would believe that he was a solitary agent, unaware of the KGB's awesome haul.

At the outbreak of the Second World War, Cairncross was posted to the Cabinet Office as a private secretary to Lord Hankey, the Chancellor of the Duchy of Lancaster. Contrary to later suggestions, Cairncross denied that he enjoyed access to atomic secrets, but he did pass on a mass of top secret cabinet papers recording the Government's political and military attitudes and decisions across the whole spectrum of daily affairs. In Hankey's office, he sensed not only the anti-Soviet atmosphere but also the continuing pro-German policies espoused by some government ministers.

In 1941, Cairncross was posted to GCHQ, the intercept station at Bletchley Park decoding secret German signals. For the KGB, Cairncross was a goldmine. Unlike other informants, Cairncross could provide pure information about the Soviets' immediate enemy. Although his first chore was to prove the Luftwaffe's order of battle, his value to the Soviets was proven in February 1943 when he handed to his Soviet contact the original flimsy papers of the intercepts, containing the full details of the Wehrmacht's summer offensive along a 1,200km front which would climax at the battle of Kursk. Initially, the Soviets undertook a series of pre-emptive air strikes but simultaneously used Cairncross's information to develop a new anti-tank shell to penetrate the new, thick German tank armour. In recognition of his critical assistance, Cairncross was awarded the Order of the Red Banner.

In Cairncross's opinion, his revelations did not render him a traitor. He was helping an ally who had been unjustly deprived of life-saving information by a right-wing clique.

By then, the Lubianka had become overwhelmed by intelligence supplied by the British traitors. To the KGB it seemed impossible that the famed British intelligence service could allow Cairncross and other officials to carry out suitcases filled with the most precious secrets from government buildings. For a brief hiatus, all of the British material, with the exception of Cairncross's, was distrusted. Then Yuri Modin, a young KGB officer, was tasked to sift all the material and recommend the best five sources, the remainder to be ignored. Modin's administrative chore, selecting Philby, Burgess, Maclean, Blunt and Cairncross, was the birth of the Ring of Five.

In 1944, exhausted by both his work at GCHQ and his double life, Cairncross was posted to MI6, the foreign intelligence service. In Section V, the counter-intelligence section, Cairncross produced under Philby's directive an order of battle of the SS. Later Cairncross would confess that he was unaware of Philby's true loyalties.

At the end of the war, Cairncross was posted to the Treasury. Although Cairncross would later claim that he ceased working for the KGB, Yuri Modin, who arrived in London in 1948 to care for Cairncross, Burgess and Blunt under cover of press attaché, tells a different story. According to Modin, "Everything flowed through the Treasury and Cairncross's information was perfect." Cairncross, as Modin wrote in his memoirs in 1994, which were shown to Cairncross prior to publication for approval, "was my favourite of the Five". Modin's only complaint was that Cairncross was "a difficult man who was impolite to the aristocrats in the Civil Service. Why he was given a job in the Civil Service has always baffled me."

Whitehall's displeasure with Cairncross was balanced by Modin's enthusiasm and, with Moscow's approval, the official was given money by the KGB to buy a car and later, in 1951, more money as a wedding present. More to the point, Modin was infuriated by Cairncross's failure to meet punctually and work a microfilm camera. The fumbling spy compensated by providing a complete collection of papers for the structure, financing and composition of Nato - even before it was created. But, in that same year, Cairncross was not forewarned by his friends of the disaster which disrupted his life.

After nearly two years' investigation MI5, Britain's counter-intelligence service, had, under the supervision of Dick White, identified Maclean as a Soviet spy. Just before Maclean's arrest, Modin had organised his escape to Moscow, but the plan misfired.

Burgess had been asked to escort Maclean to Switzerland and return to London. Instead, he continued to Moscow. When Maclean's disappearance was discovered on 28 May 1951, White froze with disbelief and his condition worsened when the unsuspected Burgess was identified as Maclean's travelling companion. With the help of the still unsuspected Blunt, MI5 entered Burgess's flat and seized a guitar case full of letters. Among them was a secret Foreign Office paper with a brief, unsigned handwritten note attached. Eventually the handwriting was identified as Cairncross's.

Up to that point, Cairncross would claim to be a friend of Burgess but unaware of his true loyalties. That opinion was supported by Modin. The KGB's compartmentalisation was so successful that Burgess, working in the Foreign Office, had persuaded Cairncross to provide him with secret papers on the grounds that the normal Whitehall channels were too slow.

Cairncross was placed under surveillance. In an operation masterminded by Anthony Simkins, Cairncross was followed through London to Ealing Common Underground station. Clearly waiting for someone, the official stood smoking and then departed. Modin had hovered nearby and departed after noticing three MI5 watchers. Back at MI5's headquarters, Simkins read the report and exclaimed, "He's a non-smoker! He was smoking to warn his Soviet contact."

If Simkins and White had stepped adroitly, the history of the Cambridge Ring and the subsequent "molehunt" would have terminated happily. Instead, before summoning Cairncross for an interview, the MI5 officers failed to gather the evidence which Bernard Hill, MI5's lawyer, firmly stipulated as necessary for a prosecution. In the interim days, Cairncross met Modin and was briefed about his behaviour in the inevitable interrogation. "I told him to admit his Communist sympathies and an innocent friendship with Burgess," Modin would later explain, "and deny any link with espionage." In the event, the intelligent Cairncross easily outsmarted Simkins and achieved practically the same success in a second interview with William Skardon, MI5's professional but flawed interrogator. After making a limited confession of carelessness with official papers, he resigned from the Civil Service. Without a confession, the Government was helpless.

Cairncross was also penniless and unemployed. Eventually, with some money received from the faithful Modin, Cairncross moved to academic life at North Western University in Chicago. Gradually, the unexposed traitor developed the remarkable skills which would establish him as an expert in Moliere and Pascal, as an authority of the Romance languages, the author of a standard work on polygamy and as a minor poet.

That pleasant life terminated in 1964 with the arrival of Arthur Martin, MI5's most outstanding investigative officer. In the aftermath of Philby's defection to Moscow, Martin had reopened the files to hunt for the Fourth and Fifth Man. To Martin's surprise, Cairncross made a full confession. Continuing to Washington, Martin received, with further surprise, a denunciation which would lead to Blunt's confession.

By then, Cairncross had moved to become an economics expert for the United Nations' Food and Agricultural Organisation in Rome, working both at headquarters and in the Third World. And it was in Rome that his secret was finally unravelled. In December 1979, Barrie Penrose, a journalist, having trawled for weeks through official lists, concluded that Cairncross was the Fifth Man and knocked on the traitor's door. Cairncross's confession was front- page news. His status was confirmed 10 years later by Oleg Gordievsky, the KGB defector.

By then Cairncross had, after a one-year imprisonment in Rome on currency charges, moved to France. Pursued by other journalists, he decided to write his own memoirs. These he completed a short time ago and are due to be published in spring 1996. Written in a different era to Philby's, they are said to contain the true confessions of a traitor.