Floud's attitude, when I began the interview, was extraordinary. He treated the matter as of little importance, and when I pressed him on Jennifer Hart's story he refused to either confirm or deny that he had recruited her.

"How can I deny it, if I can't remember anything about it?" he said repeatedly.

I was tough with him. I knew that his wife, an agoraphobic depressive, had recently committed suicide, but Floud was eager to conclude the interview, presumably lured by the scent of office. I explained to him in unmistakable terms that, since it was my responsibility to advise on his security clearance, I could not possibly clear him until he gave a satisfactory explanation for the Hart story. Still he fell back lamely on his lack of memory. The session ended inconclusively, and I asked for him to attend a further interview the following day. I did not make any progress with him, he maintaining that he had no recollection of recruiting Jennifer.

The next morning I got a message that Floud had committed suicide, apparently with a gas poker and a blanket.

My attention has been drawn to your website's entry about my father, Bernard Floud. The entry contains a number of tendentious and unproven assertions about my father, none of which can be tested since the deaths of most of those concerned, but I am concerned in particular with your repetition of the deeply distressing and totally untrue statement, repeated from "Spycatcher" by Peter Wright, that my mother - Ailsa Floud - committed suicide. My mother died in the Middlesex hospital from a long-standing lung disease.

Of those who became secret members of the Communist Party in the 1930s, quite a few refused to do any spying in later life - Michael Straight, the rich American, for example, or the civil servants Bernard Floud and Jenifer Hart...

His (Floud) profile was not that of an active underground mole at all. He had been an open supporter of Communism after the war. As a consequence, he had been 'purged' from Harold Wilson's Board of Trade in 1948 under the blacklisting procedure for civil servants. And if he had only pretended to leave the British Communist Party subsequently, his name would have been on the 55,000 files looted by MI5 from the CPGB under operation PARTY PIECE in 1955.

The major problems inside MI5 concern its relationship with the former Prime Minister Harold Wilson. As the Minister responsible for trade in the Attlee Government, he attempted to increase exports to the USSR. He constantly ran up against United States Government opposition towards any growth in such trade. Wilson felt that the United States used the hysteria of the cold war to prevent Britain from increasing its trade with the USSR. His was not a position that was likely to be viewed with favour in MI5. In fact, there was near hysteria in MI5 when he was sent to the USSR to negotiate the sale of 20 advanced jet engines. Wilson was only a junior Minister carrying out a Cabinet decision, but from that point on he was viewed with suspicion by MI5 officers.

When the unexpected death of Hugh Gaitskell led to the election of Wilson in 1963, MI5 immediately tried to recruit Wilson's campaign manager, George Caunt, to spy on the Labour leader. Shortly before the 1964 election, the FBI told MI5 that it had discovered a KGB mole who had been operating inside MI5 in the key post-war period. The fact that Sir Anthony Blunt was a KGB agent and had close connections with the Queen was certain to create a spy scandal as damaging as that of Kim Philby. Even worse for MI5 was the knowledge that it had been tipped off about Blunt's spying a decade earlier and had failed to take action. It now feared that Wilson would use the opportunity of the scandal to dismember its organisation. Sir Roger Hollis, then director-general of MI5, and Arthur Martin, head of the counter-espionage department, decided on a cover-up and did not even tell the outgoing Tory Prime Minister, Harold Macmillan. Instead, Blunt was granted immunity and was interrogated by Peter Wright, who made the position clear in Spycatcher, when he wrote: "We had strict orders from successive Director-Generals to do nothing that might provoke Blunt to go public." All that was concealed from Wilson when he became Prime Minister, and he was also not informed when Hollis and his deputy, Graham Mitchell, eventually came under suspicion as KGB moles.

Other news was kept from Wilson. In 1961, Anatoli Golitsin, a KGB defector, had arrived in the USA with all sorts of wild allegations few of which yielded anything of substance except the identity of the Admiralty spy, John Vassall. By coincidence, shortly after Wilson's election as Leader of the Opposition, Golitsin was sent to Britain to be interviewed by MI5. His agreed fee was £10,000 a month - £70,000 at today's prices - which was a considerable incentive to keep the interest of his MI5 hosts. Although he had made no mention of it during his two-year interrogation in the USA, Golitsin now told MI5 that he had heard of a KGB plot to kill the leader of a west European political party so that its man could take over. That was all that Peter Wright and other extreme right-wingers inside MI5 needed to confirm the suspicions that had been hanging around ever since the jet engine trade deal and Wilson's annual visits to the USSR while in opposition. They believed that the assassinated party leader had to be Gaitskell.

Oblivious to the suspicions of MI5 and the CIA, the new Labour Prime Minister Wilson issued instructions that MI5 was to stop tapping the telephones of Members of Parliament, although it never occurred to him that MI5 could continue to get access to the information gleaned from taps on Members of Parliament run by the CIA or GCHQ. He also instructed that MI5 should stop using Members as agents without knowing that one Tory Member, Captain Henry Kerby, had been used by MI5 to ingratiate himself with Wilson's shadow Cabinet colleague George Wigg by spying on the Tory party for Wigg. It gives one great encouragement that such people might have a greater role in law enforcement in the future.

The instructions from Wilson caused deep resentment inside MI5, where some officers retaliated by leaking damaging bits of gossip about members of Wilson's Government from MI5 files to the press. That was of course a breach of the Official Secrets Act 1911, but no one has ever been prosecuted.

MI5 believed that seven members of Wilson's Government and three other Labour Members of Parliament were either spies or at the very least security risks. Only one of those 10, Will Owen, the Member of Parliament for Morpeth, eventually turned out to be guilty. He had been taking £500 a month from Czechoslovakian intelligence in exchange for low-grade information that it could most probably have got cheaper by buying Hansard and reading the quality press. All the other names on MI5's list were completely innocent, but that did not stop MI5, in particular Peter Wright, hounding Bernard Floud, who had been devastated by the death of his wife. MI5 pursued him until he finally committed suicide in a moment of despair.

When Treasury Minister Niall MacDermot had his promotion to the Cabinet blocked following MI5 pressure on Wilson, he resigned from politics in disgust. The other seven Members on MI5's list were John Diamond, Tom Driberg, Judith Hart, Stephen Swingler, John Stonehouse, Barnet Stross and, of course, Wilson.

The MI5-inspired rumours about Wilson eventually reached the ears of former Prime Minister and Leader of the Opposition, Sir Alec Douglas-Home, who asked James Scott-Hopkins, a former MI6 officer who had become a Tory Member of Parliament, to conduct his own investigation to discover whether there was any danger of Wilson's being blackmailed.

In the summer of 1967, people from MI5 met people from the CIA, the FBI and the Australian and New Zealand security services in Melbourne, Australia, where they were addressed by Golitsin about his Wilson allegations.

Matters began to hot up when the press baron Cecil King, a long-standing MI5 agent, began to discuss the need for a coup against the Wilson Government. King informed Peter Wright that the Daily Mirror would publish any damaging anti-Wilson leaks that MI5 wanted aired, and at a meeting with Lord Mountbatten and the Government's chief scientific adviser, Solly Zuckerman, he urged Mountbatten to become the leader of a Government of national salvation. Lucky old Britain. Zuckerman pointed out that that was treason, and left the meeting. The idea came to nothing because of Mountbatten's reluctance to act.

The late Harold Wilson was not the only one under suspicion. While the right hon. Member for Old Bexley and Sidcup (Sir E. Heath) was Leader of the Opposition, the Tory Member of Parliament, Captain Henry Kerby - as I have explained, he was an MI5 agent who had ingratiated himself with George Wigg - was used to spread rumours that the right hon. Gentleman was a homosexual who had had an affair with a Swedish diplomat.

Doubts about the right hon. Member for Old Bexley and Sidcup were not confined to the more extreme elements who clustered round Peter Wright. The newly appointed head of MI5, Mr. Hanley - otherwise known as Jumbo - did not inform the right hon. Gentleman that investigations were taking place to try to determine whether Sir Roger Hollis had been a KGB agent.

The head of MI5 did not inform the Leader of the Opposition of MI5's doubts about Wilson, either, or reveal the contents of the file on Wilson that he had inherited from his predecessor, Furnival Jones, and which was kept in his safe, filed under the name "Henry Worthington".

The second factor that increased MI5's alarm at the time was the rise in trade union militancy and the swing to the left in the Labour party. Any pretext that MI5 existed to catch Russian spies went right out of the window at that point. From 1972, there was a vast growth in the sections of MI5 that were involved with domestic surveillance.

Trade unionists, peace campaigners, Cabinet Ministers and political activists in their tens of thousands became the objects of illegal telephone taps and letter intercepts. Recruitment of agents on a scale not considered necessary even at the height of the cold war meant that, by the mid-1970s, even a small group of left-wingers meeting anywhere was likely to have an MI5 agent reporting back on its activities. By the end of the 1970s, 2 million British citizens had security files held on them by MI5.

A constant drip of innuendo about Wilson's loyalty was fed by MI5 to Private Eye, and Michael Halls, the liaison officer between No. 10 and MI5, considered Marcia Williams to be a security risk and funnelled damaging smears about her and Wilson to Private Eye. As Peter Wright put it in his book: "most people in MI5 didn't have a duty to Parliament. They have a duty to the Queen... It's up to us to stop Russians getting control of the British government."

Although it is easy to dismiss some of what I have described as the work of a lunatic fringe, the views of MI5 chief Sir Michael Hanley are well known. When he was asked at a seminar for junior MI5 officers what would happen if Michael Foot became Prime Minister, he replied: "I and every other officer in the service will have to consider our position."

Other officers in MI5 did not share Hanley's sense of resignation, and 30 MI5 officers, including Peter Wright, engaged, on Wright's own admission, in 23 criminal conspiracies and committed 12 acts of treason against the elected Government of the day.

Finally, what was happening came to the attention of Sir Maurice Oldfield, then head of MI6, who took Wright to dinner at Lockets restaurant in July 1975 and asked him about the extent of the plot in MI5 against Wilson. Having heard Wright out, Oldfield told him to put MI5 chief Hanley in the picture. This Wright did the next day, and in his book he says: "Hanley . . . went white as a sheet . . . he was learning that half of his staff were up to their necks in a plot to get rid of the Prime Minister."

When we consider that record, we realise that it involves more than one or two eccentrics such as Peter Wright, and that there is clearly a culture of extreme right-wing politics in MI5, which has been there throughout this century. Occasionally there are brought to the surface very deep links with individual Conservative Members of Parliament who have been able to use MI5, disinformation and black propaganda to damage Labour Members of Parliament and Governments.

One Girl's War poses no threat to national security; if other books do and if the Government wishes to take action against them, then that is their business, not ours. The content of One Girl's War has to do solely with events which took place over forty years ago, and we believe that it should be considered for what it is, not for what other books might be.

The Government's attempt to suppress One Girl's War is part of a larger project to keep from the British public any information about the operations of the intelligence services and thus to render impossible any public debate on the matter. In the 1960s and 1970s most countries of the western world gradually liberalised public access to information; in the 1980s Margaret Thatcher's Governments have sought to reverse that trend. There are general ideological reasons for this, and there are particular reasons.

Between 1974 and 1976 a coalition of right-wing Conservative politicians and elements of the armed forces and of the intelligence services worked secretly to subvert the elected Labour Government led by Harold Wilson. It is not suggested that this coalition was responsible for the demise of the Wilson Government and the installing of Margaret Thatcher as Prime Minister. But the whole notion of such secret activity involving state security services in efforts to undermine the elected Government runs so sharply against the general perception of British democratic tradition that it is hardly surprising if the Thatcher Government is determined to ensure that the full story is never told.

Christopher Andrew's book has been sanitised in "an extensive clearance process involving other departments and agencies". The present head of MI5, Jonathan Evans, writes that information has been censored not only for "national security" but also "if its publication would be inappropriate for wider public interest reasons". So readers are asked to take much on trust as they plough through reams of uncheckable footnotes merely labelled "security service archives" or "recollections of a former security service officer".

When it comes to the Wilson affair, Andrew's scholarship appears to slip. He repeats insistently the MI5 party line that there never was misbehaviour against Wilson or his ministers by "the Service", and that it was all mere conspiracy theories. Yet he withholds the fact that the cabinet secretary Lord Hunt authoritatively confirmed the central allegation. Hunt, who conducted a secret inquiry, said in August 1996: "There is absolutely no doubt at all that a few, a very few, malcontents in MI5 . . . a lot of them like Peter Wright who were rightwing, malicious and had serious personal grudges – gave vent to these and spread damaging malicious stories about that Labour government."

Separately, Andrew is silent on the well-documented case in which Wilson's treasury minister, Niall McDermot, was driven from office in 1968 when Patrick Stewart of MI5 accused his Russian-born wife of having KGB contacts. It was a most unpleasant miscarriage of justice.

Andrew does, however, write his own chapter on Wilson. He calls it "The 'Wilson Plot'". Puzzled readers may conclude that he is seeking, bu using those quotation marks, to explode the 1988 book of that title, written by this reviewer who coined the phrase. Yet no references at all follow in the footnotes or bibliography. It has become an Un-book, and the reader is not able to consider its countervailing evidence.

Nonetheless, it turns out in the end that Andrew must be using a sort of code. To one's surprise, underneath the MI5-approved bluster against "conspiracy theories", there lurks the real story in obscure footnotes and cryptic mentions. Andrew has in fact substantiated the thesis of the "Wilson plot", and more besides. It transpires that there is even more damning material in the MI5 files than was ever realised.

For example, the Labour MP Bernard Floud was indeed bullied with false allegations that he was a communist while in a state of grief after his wife's death. Thanks to the files, Andrew exonerates him and confirms that Peter Wright lied about the relevant dates of interrogations to try to make Floud look guilty. Dramatically, it is confirmed that there was indeed a secret Wilson MI5 file, under the pseudonym "Worthington". (One of my own errors is corrected. It was "Norman" not "Henry" Worthington.) The file was opened in 1947, when a communist civil servant spoke approvingly about Wilson on a tapped phone. It detailed Wilson's postwar trips to Moscow working for a timber firm (which we knew of) and his secretly observed encounters in London around 1955 with an undercover KGB man called Skripov (which we didn't).