The Regular Reserve is the component of the military reserve of the British Armed Forces whose members have formerly served in the "Regular" (full-time professional) forces. Other components of the Reserve are the Volunteer Reserves and the Sponsored Reserves. The Regular Reserve largely consists of ex-Regular personnel who retain a statutory liability for service and are liable to be recalled for active military duty "in case of imminent national danger or great emergency". It also consists of a smaller number of ex-Regulars who serve under a fixed-term reserve contract (similar in nature to the Volunteer Reserves) and are liable for reporting, training and deploying on operations.

Since April 2013, Ministry of Defence publications no longer report the entire strength of the Regular Reserve, instead, only Regular Reserves serving under the fixed-term reserve contract are counted. As of 2014, they had a strength of 45,110 personnel. Of those, approximately 2,450 were serving alongside the Regular military in active service.

The Regular Reserve of the British Army was originally created under the Reserve Force Act 1859 (22 & 23 Vict. c. 42) by Secretary of State for War Sidney Herbert, and re-organised under the Reserve Force Act 1867 (30 & 31 Vict. c. 110).

From the end of the Napoleonic Wars until 1847, men were enlisted for twenty-one years, practically for life. Thereafter, the term of enlistment was ten years, then twelve years. On completion of their enlistment, soldiers had the choice between accepting discharge without pension or to extend so as to accumulate 21 years of service. After many years with no trade other than that of soldiering, more than half of all discharged soldiers chose to re-enlist immediately. Of those who took a voluntary discharge, fully one in five signed on again within six months.

The Army's existing system of enlistment therefore produced an army of experienced or even veteran soldiers, but no class of reserves that could be recalled to serve in case of a national emergency. Under the Reserve Force Act 1867 (30 & 31 Vict. c. 110), a "First Class Army Reserve" had been created, of soldiers released from active service who had not completed their terms of service, to have an establishment of 20,000 men in theory. In practice, as of 1868, only 2,033 were in this body of men. The "Second Class Army Reserve" was to consist of army pensioners and of discharged soldiers having at least five years regular service. The First Class Army Reserve was liable for overseas service in the event of war, whereas the Second Class Army Reserve was for home service to defend against invasion. Cardwell therefore brought before Parliament the idea of "short service". The Army Enlistment Act 1870 (33 && 34 Vict. c. 67) allowed a soldier to choose to spend time in the reserves after service with the colours. As to the proportion of time spent on active service with the colours versus the balance in the reserve, this was to be laid down from time to time by the Secretary of State for War.

Upon transferring to the Army Reserve, for the remainder of their 12 years, the soldier would be transferred to Section A or Section B, with Section C being subsumed into Section B in 1904. Section A was voluntary and limited, each infantry regiment being permitted about 50 men on their books. Section B was the normal destination for the balance of the 12 years enlistment.

Among the other benefits, this thereby enabled the British Army to have a ready pool of recently-trained men to draw upon in an emergency. The name of the Regular Reserve (which for a time was divided into a First Class and a Second Class) has resulted in confusion with the Reserve Forces, which were the pre-existing part-time, local-service home-defence forces that were auxiliary to the British Army (or Regular Force), but not originally part of it: the Honourable Artillery Company, Yeomanry, Militia (or Constitutional Force) and Volunteer Force. These were consequently also referred to as Auxiliary Forces or Local Forces.

In 1881, under the Childers Reforms short service was increased to seven years with the colours, and five with the reserve, of the twelve-year enlistment period that the Cardwell Reforms had introduced. This also introduced the ability for time-served soldiers to extend service in the reserve by four years, albeit classed as the second division, or Section D, of the First Class Army Reserve.