Facts

The Security Service, also known as MI5 (Military Intelligence, Section 5) is the United Kingdom's domestic counter-intelligence and security agency, and is part of its intelligence machinery alongside the Secret Intelligence Service (MI6), Government Communications Headquarters (GCHQ), and Defence Intelligence (DI). MI5 is directed by the Joint Intelligence Committee (JIC), and the service is bound by the Security Service Act 1989. The service is directed to protect British parliamentary democracy and economic interests, and counter terrorism and espionage within the United Kingdom (UK).

Source: Wikipedia