The Security Service, also known as MI5, originated in 1909 as the internal arm of the Secret Service Bureau, under Army Captain (later Sir) Vernon Kell, tasked with countering German espionage. In 1931 it assumed wider responsibility for assessing threats to national security which included international communist subversion and, subsequently, fascism. In 1952, in the early stages of the Cold War, the work of the Service and the responsibility of the Director General were defined in a Directive many of whose provisions were later incorporated in the Security Service Act 1989.
Today the Security Service Act forms the statutory basis for the Service, which is placed under the authority of the Home Secretary. The Act also sets out the functions of the Service, as well as certain controls and oversight arrangements. As the UK's domestic security intelligence agency the Service's purpose is to protect the State against substantial, covertly organised threats, primarily from terrorism, espionage and subversion. Most recently, since the passing of the Security Service Act 1996, its role has been expanded to provide support to law enforcement agencies in the field of organised crime.
The Director General of the Security Service is Eliza Manningham-Buller, who took office in May 2002. The Deputy Director-General is Mr Fraser Wilson.