The name by which the counter-espionage department of the War Office was known from January 1916.

Britain's defensive security service dealing with internal intelligence and counter-intelligence was set up in 1909. During the Second World War MI5 (the informal name for the security service) dealt with all aspects of internal security, including the running of the double-cross system. After the war it was proved that the MI5 knew of and dealt with all German agents in Britain.

Military Intelligence 5 - Covername of the Security Service