In 1907 Major Vernon Kell become Director of the Home Section of the Secret Service Bureau with responsibility for investigating espionage, sabotage and subversion within and without Britain. In 1911, a new section, headed by Captain Mansfield Cumming, became responsible for for secret operations outside Britain.

In the First World War Cumming's unit became known as MI6. Agents who worked for MI6 during the war included John Buchan and Somerset Maugham. Working closely with Vernon Kell of MI5 and Sir Basil Thomson of the Special Branch, on the outbreak of war 22 German agents were arrested. Eleven men were executed, as was Sir Roger Casement, who was found guilty of treason in 1916.

During the First World War MI6 provided money to prop up the the government of Alexander Kerensky. After the successful Russian Revolution British diplomatic representation was withdrawn from Moscow. However, an unofficial MI6 mission remained which funded anti-Bolshevik groups in sabotage and subversion. The most active MI6 agent in Russia was Sidney Reilly. After a plot to assassinate Lenin failed, Reilly was forced to flee the country.

After the war the government cut back on expenditure on the Secret Service. As a result Smith-Cumming lost MI6 stations in Madrid, Lisbon, Zurich and Luxembourg.

In 1926 Admiral Hugh Sinclair became the new head of MI6. Sinclair was a strong opponent of communism and argued that telegrams sent by Maxim Litvinov showed that Russia was financing Sinn Fein. Later it was revealled that these telegrams were forgeries.

In 1932 Compton Mackenzie published Greek Memories, an account of his experiences as a MI6 officer during the First World War. In the book he disclosed for the first time that Secret Intelligence Service (SIS) existed. He even revealed that the first Director-General of the organization was Mansfield Cumming. In one passage he referred to the organization as being "scores of under-employed generals surrounded by a dense cloud of intelligence officers sleuthing each other."

The book was immediately withdrawn and all remaining copies were destroyed. Mackenzie was fined £100 for breaching the Official Secrets Act. Mackenzie's actions were now monitored by MI5. One agent claimed that he was overhead telling a journalist from the Daily Telegraph that MI5 was an inefficient organization and that Vernon Kell and his staff were incompetent.

Hugh Christie an MI6 agent working based in Berlin, met with Hermann Goering on 3rd February 1937. He immediately reported his conversation with Goering and included information that Germany intended to take control of Austria and Czechoslovakia. He also told Christie that Germany mainly wanted "a free hand in Eastern Europe."

In March 1938 Hugh Christie told the British government that Adolf Hitler would be ousted by the military if Britain joined forces with Czechoslovakia against Germany. Christie warned that the "crucial question is 'How soon will the next step against Czechoslovakia be tried?' ... The probability is that the delay will not exceed two or three months at most, unless France and England provide the deterrent, for which cooler heads in Germany are praying."

International tension increased when Adolf Hitler began demanding that the Sudetenland in Czechoslovakia should be under the control of the German government. In an attempt to to solve the crisis, the heads of the governments of Germany, Britain, France and Italy met in Munich in September, 1938.

On 29th September, 1938, Adolf Hitler, Neville Chamberlain, Edouard Daladier and Benito Mussolini signed the Munich Agreement which transferred to Germany the Sudetenland, a fortified frontier region that contained a large German-speaking population. When Eduard Benes, Czechoslovakia's head of state, who had not been invited to Munich, protested at this decision, Chamberlain told him that Britain would be unwilling to go to war over the issue of the Sudetenland.

Several figures in MI6 were sympathetic to the government of Nazi Germany. Wing Commander Frederick Winterbotham head of MI6's air section, argued that Britain and Germany should unite against the Soviet Union. In March 1939 Hugh Sinclair rejected evidence that Germany planned to go to war against Britain as "alarmist rumours put forward by Jews and Bolsheviks."

In July 1939 Stewart Menzies went to Warsaw to supervise the capture of the Enigma coding device. On his return to Britain he became the Director General of MI6.

On 18th August, 1939, Hugh Christie, MI6's agent in Berlin, told the British government that Adolf Hitler had decided to launch an attack Poland in September. Once again Neville Chamberlain decided to ignore this information.

After the outbreak of the Second World War MI6 recruited a large number of part-time agents including Graham Greene, Malcom Muggeridge and Kim Philby. Over the next few years Menzies attempted to persuade Admiral Wilhelm Canaris, the head of Abwehr, the German Secret Service, to become a British agent. This strategy was unsuccessful.

In October 1944 Stewart Menzies appointed Kim Philby as head of Section IX (Soviet Affairs). After the war Philby was responsible for monitoring Soviet espionage. In this role he was able to protect other Soviet agents such as Guy Burgess, Donald Maclean and Anthony Blunt.

In September 1945, a Russian diplomat, Constantin Volkhov, approached the British vice-consul in Istanbul with information about three Soviet agents working in the Foreign Office and the counter-espionage service in London. Kim Philby was able to tell the KGB who quickly arrested Volkhov and took him back to the Soviet Union.

In 1949 Kim Philby became the MI6 liaison officer in Washington. In this post he was able to discover that SIS planned to overthrow Enver Hoxha, the communist dictator of Albania. Philby was able to communicate this information to the Soviet Union and the Albanians involved in the conspiracy were arrested and executed.

In 1950 Stewart Menzies and John Sinclair discussed the possibility of Philby becoming the next Director General of the MI6. Dick White was asked to produce a report on Philby. He asked Arthur Martin and Jane Archer to carry out an investigation into his past. They became concerned about how quickly changed from a communist sympathizer to a supporter of pro-fascist organizations. They also discovered that the description of the mole provided by Walter Krivitsky and Igor Gouzenko was close to that of Philby while he was working in Spain as a journalist. It was now decided that Philby could in fact be a double-agent.

When Guy Burgess and Donald Maclean defected in 1951 Kim Philby became the chief suspect as the man who had tipped them off that they were being investigated. Philby was interrogated by MI6 but they cleared him of being part of a spy ring. However, the CIA insisted that he should be recalled to London and later that year he left MI6.

In 1951 Mohammed Mussadeq, took power in Iran and nationalised the Anglo-Iranian Oil Company, Britain's largest overseas asset and the world's biggest oil-producer. The British foreign minister, Sir Anthony Eden, approved a SIS plot to overthrow Mussadeq. The following year MI6 agent George Young helped to organize protests demonstrations against the government in Iran. In August 1953 over 300 people died during a riot in Teheran. Mussadeq resigned and was replaced by the SIS candidate, the Shah of Iran. Later George Young was involved in a plot to assassinate Gamal Nasser.

In 1953 Major-General John Sinclair, a former Director of Military Intelligence, replaced Major General Stewart Menzies as Director-General of MI6. During the 1950s MI6's E Branch, was involved in countering campaigns of insurgency in Malaya, Borneo, Kenya and Cyprus.

On 23rd October, 1955, the newspaper, New York Sunday News, reported that Kim Philby was a Soviet spy. Two days later Marcus Lipton asked Sir Anthony Eden in the House of Commons: "Has the prime minister made up his mind to cover up at all costs the dubious third-man activities of Mr. Harold Philby". Eden refused to reply but, Harold Macmillan, the foreign secretary, issued a statement a couple of days later: "While in government service he (Philby) carried out his duties ably and conscientiously, and I have no reason to conclude that Mr Philby has at any time betrayed the interests of his country, or to identify him with the so-called 'Third Man', if indeed there was one."

Philby now called a press conference where he denied he was a spy. He added that "I have never been a communist and the last time I spoke to a communist knowing he was one, was in 1934". Philby accused of Lipton of lying and challenged him to repeat his claims outside the protection of the House of Commons. Lipton was forced to issue a statement where he withdrew his comments. Philby now moved to the Middle East where he worked as a foreign correspondent for The Observer and the The Economist. He also continued to work as a part-time agent of MI6.

SIS were involved in several attempts to overthrow foreign governments. This included one plot in 1955 to have Ho Chi Minh murdered. That ended in failure but the MI6 agent George Young worked closely with the CIA in the removal of Patrice Lumumba in the Congo.

In April 1956 Commander Lionel Crabb, an underwater sabotage expert, disappeared on a secret mission to investigate the Russian cruiser Ordkhonikidze. This created a diplomatic row as the ship had brought over Nikita Khrushchev and Nikolai Bulganin on a goodwill mission to Britain.

Sir Anthony Eden, the British prime minister was furious when he discovered about the MI6 operation that had taken place without his permission. Eden forced the Diretor-General of MI6, Major-General John Sinclair, to resign. He was replaced by Sir Dick White, the head of MI5. As MI5 was considered by MI6 to be an inferior intelligence service, this was the severest punishment that could be inflicted on the organization.

In December 1961, Anatoli Golitsin, a KGB agent, working in Finland, defected to the CIA. He was immediately flown to the United States and lodged in a safe house called Ashford Farm near Washington. Interviewed by James Angleton, Golitsin supplied information about a large number of Soviet agents working in the West.

In these interviews Golitsin argued that as the KGB would be so concerned about his defection, they would attempt to convince the CIA that the information he was giving them would be completely unreliable. He predicted that the KGB would send false defectors with information that contradicted what he was saying.

In June 1962 Yuri Nosenko made contact with the CIA in Geneva. He was deputy chief of the Seventh Department of the KGB. His main responsibility was the recruitment of foreign spies. He like Golitsin, provided evidence that John Vassall was a Soviet agent. However, most of his evidence undermined that given by Golitsin. This included Golitsin's claim that a senior figure in the Admiralty was a spy.

Arthur Martin, head of MI5's D1 Section, went to to interview Anatoli Golitsin in America. Golitsin provided evidence that suggested that Kim Philby had been a member of a Ring of Five agents based in Britain. The same spy ring that had included Guy Burgess and Donald Maclean.

Philby was questioned once more by MI6 about being a Soviet agent. Aware that he was in danger of being arrested, on 23rd January, 1963, Philby fled to the Soviet Union. In his book, My Silent War (1968), Philby admitted that he had been a Soviet spy for over thirty years.

Anthony Blunt also confessed his crimes but was granted immunity from prosecution and continued as Surveyor of the Queen's Pictures. Blunt was not exposed as a spy until 1979.

Other Director-Generals of MI6 have included John Rennie (1968-73), Maurice Oldfield (1973-78), Arthur Franks (1979-82), Colin Figures (1982-85), Christopher Curwe (1985-89), Colin McColl (1989-94) and David Spedding (1994-99).