MI5 is responsible for protecting the UK, its citizens and interests, at home and overseas, against threats to national security. The role of MI5, as defined in the Security Service Act 1989, is “the protection of national security and in particular its protection against threats such as terrorism, espionage and sabotage, the activities of agents of foreign powers, and from actions intended to overthrow or undermine parliamentary democracy by political, industrial or violent means”.
MI5 was renamed to the “Security Service” in 1931 when it merged with Scotland Yard’s Special Section, which had similar responsibilities for domestic intelligence. It is no longer part of the UK military establishment, although they do still contribute – along with the military – to the overall national security of the UK. They continue to use the name “MI5” as shorthand for their official name, the Security Service.