MI numbers in the context of "Secret Intelligence Service"

The Secret Intelligence Service (SIS), commonly known as MI6 (Military Intelligence, Section 6), is the foreign intelligence service of the United Kingdom, tasked mainly with the covert overseas collection and analysis of human intelligence on foreign nationals in support of its Five Eyes partners. SIS is one of the British intelligence agencies and the Chief of the Secret Intelligence Service (known as "C") is directly accountable to the Foreign Secretary.

Formed in 1909 as the foreign section of the Secret Service Bureau, the section grew greatly during the First World War, officially adopting its current name around 1920. The name "MI6" originated as a convenient label during the Second World War, when SIS was known by many names. It is still commonly used today. The existence of SIS was not officially acknowledged until 1994. That year the Intelligence Services Act 1994 (ISA) was introduced to Parliament, to place the organisation on a statutory footing for the first time. It provides the legal basis for its operations. Today, SIS is subject to public oversight by the Investigatory Powers Tribunal and the Intelligence and Security Committee of Parliament.

The Security Service, commonly 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 to counter terrorism and espionage within the United Kingdom. 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.

The Security Service is derived from the Secret Service Bureau, founded in 1909. At the start of the First World War, it was responsible for the arrest of enemy spies, or suspected enemy spies. Throughout the First World War, Germany continually attempted to infiltrate Britain, but MI5 was able to identify most, if not all, of the agents dispatched. During the Second World War, it developed the Double-Cross System. This involved attempting to "turn" captured agents wherever possible, and use them to mislead enemy intelligence agencies.