The British Militia was the principal military reserve force of the United Kingdom of Great Britain and Ireland. Militia units were repeatedly raised in Great Britain during the Victorian and Edwardian eras for internal security duties and to defend against external invasions. The British Militia was transformed into the Special Reserve under the Territorial and Reserve Forces Act 1907, which integrated all militia formations into the British Army.

The Special Reserve was established on 1 April 1908 with the function of maintaining a reservoir of manpower for the British Army and training replacement drafts in times of war. Its formation was part of the military reforms implemented by Richard Haldane, the Secretary of State for War, which also created the Territorial Force. Haldane originally intended that the Militia would provide the reserve, but opposition from its representatives forced him to abolish it and create the Special Reserve instead. Only 60 per cent of the Militia transferred into the new reserve, and it was consistently under strength, particularly in officers. Reservists enlisted for a six-year term of service, and had to undergo six months of basic training on recruitment and three to four weeks training annually.