Is it 'MI5' or the 'Security Service'?

The history of our name

We have held a variety of names, most notably MI5, since we were established in 1909. Here’s a summary: 


We began as the Home Section of the Secret Service Bureau, under Captain Vernon Kell. (See "The establishment of the Secret Service Bureau"). The Bureau was placed under the nominal supervision of the Directorate of Military Operations of the War Office, the predecessor of today's Ministry of Defence. The branch of the Directorate of Military Operations responsible for the Secret Service Bureau was called MO5. At the time, those aware of our existence sometimes referred to us as the Counter-Espionage Bureau or the Special Intelligence Bureau. 


The Secret Service Bureau was absorbed into the War Office for the duration of the World War 1. It becomes part of section 5 of the Directorate of Military Operations and is given the name MO5(g), with (g) simply reflecting its placement in the War Office among other branches with letters to their name. 


MO5(g) was moved to form part of the newly established Directorate of Military Intelligence within the War Office. It became section 5 of the Directorate of Military Intelligence. For the first time, we were known as MI5.


MI5 was renamed as the Defence Security Service, but this was short-lived. 


The Defence Security Service becomes the Security Service, the name by which we are still known today. This marked our separation from the War Office and the beginning of an expanded remit to include counter-subversion and counter-espionage beyond the government and military. However, MI5 has prevailed and it is still used as a short alternative to our official name which is used in the legislation which governs the work we do.