After the Bolshevik coup d'etat of October 1917, MI5 began to work on the threats from Communist subversion within the Armed Services, and sabotage to military installations. These threats attained additional importance in the mid-1920s, with the publication of the infamous Zinoviev Letter, in which the Comintern (a Soviet-run organisation which aimed to coordinate Communist effort worldwide) appeared to instruct the British Communist Party to forment insurrection. The letter caused a considerable uproar. An investigation by a Foreign Office historian recently concluded that the letter was almost certainly a forgery produced by anti-communists in Latvia, but was not, as has often been alleged, the result of an MI5/MI6 conspiracy to bring down the Labour Government of the day - though it was recognised that one or two staff, acting on their own initiative, might have been responsible for leaking the letter to the press. On 15 October 1931 formal responsibility for assessing all threats to the national security of the United Kingdom, apart from those posed by Irish terrorists and anarchists, was passed to MI5. This date marked the formation of the Security Service, although the title MI5 has remained in popular use to this day.

Following Hitler's rise to power, the new Service had to face the threat of subversion from Fascists. However, at the time of the outbreak of the Second World War it was ill-equipped for its many tasks, which included counter-espionage; monitoring of enemy aliens and advising on internment; vetting checks for government departments; visiting firms engaged in war work to advise them on security measures against espionage and sabotage; and dealing with reports by members of the public concerning suspicious activity. In early 1939 the Service's strength stood at only 30 officers and its surveillance section comprised just 6 men. To make matters worse, in September 1940 many of its records were destroyed by German bombing.

Internment at the outbreak of the War effectively deprived the Germans of all their existing agents. Moreover, when German intelligence records were studied after 1945, it was found that all of the further 115 or so agents targeted against Britain during the course of the War had been successfully identified and caught, with the exception of one who committed suicide before capture. Some of these agents were 'turned' by the Service and became double agents who fed false information to the Germans concerning military strategy throughout the War. This was the famous 'Double Cross' system. This highly effective deception contributed to the success of the Allied Forces landing in Normandy on 'D Day' in June 1944.

In 1952 the Prime Minister, Winston Churchill, deputed his personal responsibility for the Security Service to the Home Secretary, Sir David Maxwell Fyfe, who issued a Directive describing the Service's tasks and setting out the role of the Director General. This Directive provided the basis for the Service's work until 1989, when the Security Service Act placed the Service on a statutory footing for the first time.

Following the defeat of Nazi Germany and the advent of the Cold War, the Service turned its attention to the threat from the Soviet Union. It had for some time already been focused on the activities of the Communist Party of Great Britain which, at its peak in the early 1940s, had 55,000 members. In March 1948 the Prime Minister, Clement Attlee, announced that Communists as well as Fascists were to be excluded from work 'vital to the security of the state'. This was achieved through the setting up of the vetting system, which the Service was charged to support. The cases of Philby, Burgess and MacLean, in particular, showed how effective the Russian Intelligence Service had been before the War in recruiting ideologically-motivated spies in Britain.

In the 1960s, the successful identification of a number of spies - including George Blake, an officer of the Secret Intelligence Service; the Portland spy ring; and John Vassall, an employee at the Admiralty recruited by the KGB in Moscow - illustrated the need for still greater counter-espionage efforts. Lord Denning's report into the Profumo Affair in 1963 revealed publicly for the first time details of the Service's role and responsibilities. This period of its history culminated in the mass expulsion from the UK in 1971 of 105 Soviet personnel, which severely weakened Russian intelligence operations in London.

By the late 1970s, the Service's resources were being redirected from work on subversion into international and Irish terrorism. The Service's counter-terrorist effort had begun in the late 1960s in response to the growing problem of Palestinian terrorism. Major incidents, including the terrorist sieges at the Iranian Embassy in London in 1980 and the Libyan People's Bureau in 1984, tested the Service's developing procedures and links with other agencies. During this period, the Service played a leading role in establishing an effective network for cooperation on terrorism among Western security and intelligence services.

Major changes in the focus of the Service's work took place in the early 1990s with the end of the Cold War. The threat from subversion had diminished, and the threat from espionage, though it persisted, required less of the Service's effort. Terrorism, however, had not abated. In October 1992 responsibility for leading the intelligence effort against Irish republican terrorism on the British mainland was transferred to the Service. In this new work, the Service was able to draw on the experience it had gained in the 1970s and 1980s in running long-term intelligence operations to counter other manifestations of terrorism. Between 1992 and April 1998 the Service's work with the police against Irish republican terrorism resulted in 18 convictions for terrorist-related offences. Many intended terrorist attacks, including large city-centre bombings, were prevented.

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