MI5 (Security Service)

Tasks

According to MI5, its role as defined in the Security Service Act 1989 covers the protection of national security, including protecting the United Kingdom against terrorism, espionage, sabotage and actions that undermine parliamentary democracy.¹

MI5 lists its areas of work as counter-terrorism, counter-espionage (many states try to gain access to British information and material), cyber security, counter-proliferation of weapons of mass destruction, and protective security.¹

History and names

MI5 states that it was founded in 1909 as the Secret Service Bureau, and that its founder was Captain (later Major General) Vernon Kell. From these modest beginnings grew both MI5 and MI6.²

The service began as the Home Section of the Secret Service Bureau, attached to the War Office’s Directorate of Military Operations as MO5; during the First World War it operated as MO5(g). It took the name MI5 as section 5 of the new Directorate of Military Intelligence. It was later briefly called the Defence Security Service and then the Security Service – the name used in legislation, which marked its separation from the War Office. The abbreviation MI5 remained in use as a shorter name.³

According to MI5, at the start of the First World War the service played the leading role in catching most of Imperial Germany’s intelligence agents operating in Britain.²

Parliamentary oversight

MI5, SIS and GCHQ are overseen by the Intelligence and Security Committee of Parliament (ISC), created by the Intelligence Services Act 1994 and reformed by the Justice and Security Act 2013. The Committee also oversees, among others, Defence Intelligence in the Ministry of Defence.⁴

The ISC has nine members drawn from both Houses of Parliament. They are appointed by the Houses from candidates nominated by the Prime Minister after consulting the Leader of the Opposition; the Chair is chosen by the members.⁴

Archives

In January 2025 the UK National Archives released more than a hundred MI5 files on the Cambridge Five.⁵