The Militia of the United Kingdom were the military reserve forces of the United Kingdom after the Union in 1801 of the former Kingdom of Great Britain and Kingdom of Ireland.
The militia was transformed into the Special Reserve by the Territorial and Reserve Forces Act 1907.
For the period before the creation of the United Kingdom, in the home nations and their colonies, see Militia (Great Britain).
A separate voluntary Local Militia was created in 1808 before being disbanded in 1816
By 1813 the British Army was experiencing a shortage of manpower to maintain their battalions at full strength. Some consideration was given to recruiting foreign nationals, however on 4 November 1813 a bill was introduced to parliament to allow Militia volunteers to serve in Europe. In the event only three battalions were raised and these were sent to serve under Henry Bayly arriving in Bordeaux on 12 April 1814.
After the Napoleonic Wars the Militia felling virtual abeyance, although regimental colonels and adjutants continued to appear in the Army List. Although muster rolls were prepared as late as 1820, the element of compulsion was abandoned, and the militia was transformed into a volunteer force.
The Militia was revived by the Militia Act of 1852, enacted during a period of international tension. As before, units were raised and administered on a county basis, and filled by voluntary enlistment (although conscription by means of the Militia Ballot might be used if the counties failed to meet their quotas). It was intended to be seen as an alternative to the army. Training was for 56 days on enlistment, then the recruits would return to civilian life but report for 21–28 days training per year. The full army pay during training and a financial retainer thereafter made a useful addition to the men's civilian wage. Of course, many saw the annual camp as the equivalent of a paid holiday. The militia thus appealed to agricultural labourers, colliers and the like, men in casual occupations, who could leave their civilian job and pick it up again. The militia was also a significant source of recruits for the Regular Army, where men had received a taste of army life. An officer's commission in the militia was often a 'back door' route to a Regular Army commission for young men who could not obtain one through purchase or gain entry to Sandhurst.