1883 - Metropolitan Police 'Special Branch' established to monitor Irish dissidents.

1909- Secret Service Bureau set up during hysteria over German spies.

1911 - Revised Official Secrets Act. Winston Churchill is Home Secretary.

1914 - Defence of the Realm Act.

1916 - Secret Service Bureau relaunched as part of Directorate of Military Intelligence. Renamed MI5.

MI5 is now officially known as the Security Service

1926 - General Strike. MI5 and Special Branch portray Strike as well-organised Soviet plot. Communism targeted by Security Services.

1931 - Formal responsibility for assessing threats to national security, apart from those posed by Irish dissidents and anarchists, passed to MI5, now officially known as the Security Service.

1930s - MI5 investigated Mosley and British Union of Fascists.

In early 1939 the Service's strength stood at only 30 officers and its surveillance section comprised just six men.

1941 - Sir David Petrie appointed first Director General of MI5 and given resources to rebuild the organisation.

1944 - MI5 pull off coup with double cross system of spies which contributed to success of 'D Day' landing in Normandy.

MI5 intercept Nazi intelligence by code-breaking at Bletchley Park.