MI5

MI5, officially the Security Service, 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 to counter terrorism and espionage within the United Kingdom (UK). Within the civil service community, the service is colloquially known as Box, or Box 500, after its official wartime address of PO Box 500; its current address is PO Box 3255, London SW1P 1AE.