The article lists British Army reserve brigades in World War I. At the start of the war volunteers in the vast majority of cases joined their local infantry regiment's reserve battalion. As the army expanded rapidly, further reserve battalions and brigades were formed. After conscription was introduced in 1916 the existing regimental system could not cope with the large influx of recruits and the existing reserve brigades were incorporated into the Training Reserve, with an official complement of over 209,000 soldiers, in addition to the regimental training units.

On 6 August 1914, less than 48 hours after Britain's declaration of war, Parliament sanctioned an increase of 500,000 men for the Regular British Army, and the newly-appointed Secretary of State for War, Earl Kitchener of Khartoum issued his famous call to arms: 'Your King and Country Need You', urging the first 100,000 volunteers to come forward. This group of six divisions with supporting arms became known as Kitchener's First New Army, or 'K1'. 'K2' and 'K3', followed shortly afterwards.^[1] In addition, recruits flooded into the drill halls of the part-time Territorial Force (TF), which had largely volunteered for overseas service.^[2] The flood of volunteers overwhelmed the ability of the army to absorb and organise them, and by the time the Fifth New Army ('K5') was authorised on 10 December 1914, many of the units were being organised as 'Pals battalions' under the auspices of mayors and corporations of towns up and down the country. Many of these pals who had lived and worked together, joined up and trained together and were allocated to the same units. The policy of drawing recruits from amongst the local population ensured that, when the Pals battalions suffered casualties, whole towns, villages, neighbourhoods and communities back in Britain were to suffer disproportionate losses.^[3][4][5]

By the end of August 1914, 300,000 men had signed up to fight, and another 450,000 had joined up by the end of September.^[6] Voluntary recruitment remained fairly steady through 1914 and early 1915, but it fell dramatically thereafter. Conscription for single men was introduced in January 1916. Four months later, in May 1916, it was extended to all men aged 18 to 41.^[7] The Military Service Act March 1916 specified that men from the ages of 18 to 41 were liable to be called up for service in the army, unless they were married (or widowed with children), or served in one of a number of reserved occupations, which were usually industrial but which also included clergymen and teachers. This legislation did not apply to Ireland, despite its then status as part of the United Kingdom (but see Conscription Crisis of 1918).^[8] By January 1916, when conscription was introduced, 2.6 million men had volunteered for service, a further 2.3 million were conscripted before the end of the war; by the end of 1918, the army had reached its peak strength of 4 million men.^[5]

Before the war I British Army infantry regiments trained their recruits at their home depots from where they were drafted to one of their regiment's Regular Army battalions. On the outbreak of the war the part-time Special Reserve (SR; formerly Militia) battalions were mobilised to supply these reinforcement drafts from Reservists, Special Reservists, returning wounded, and new recruits. The voluntary TF was also mobilised, and after its units volunteered for overseas service in August 1914 they began forming 2nd Line battalions to train their own recruits. By May 1915 the 2nd Line was also being prepared for overseas service, and the TF began forming 3rd Line (in some cases 4th Line) units for training. Meanwhile, the battalions of the 1st, 2nd and 3rd New Armies ('K1', 'K2' and 'K3' of 'Kitchener's Army') were quickly formed at the regimental depots. The SR battalions also swelled with new recruits and were soon well above their establishment strength. By the autumn the depots were overwhelmed, and on 8 October 1914 each SR battalion at its coast defence station was ordered to use its surplus to form a service battalion of the 4th New Army ('K4'). These were organised into six new divisions (30th–35th) divided into 18 brigades (89th–106th). However, on 10 April 1915 the War Office decided to convert the K4 battalions into reserve units, to provide drafts for the K1–K3 battalions in the same way that the SR was doing for the Regular battalions. The K4 divisions were scrapped and their brigades renumbered as 1st–18th Reserve Brigades (the original brigade and division numbers were re-allocated to the 'Pals battalions' of K5). Later the depot companies of the K5 battalions were combined into reserve battalions, and grouped into 19th–26th Reserve Brigades.^{[2][9][10][11]}

On 1 September 1916 the whole training system was centralised with the formation of the Training Reserve (TR). The K4 and K5 reserve battalions lost their regimental affiliation and were redesignated as 1st–112th TR battalions, to which all recruits not required for the regimental reserves (SR and TF) were posted. They were issued with 'General Service' badges and could be drafted to any regiment, but the training officers and non-commissioned officers remained part of their parent regiments. The reserve brigades had now been concentrated at large training camps, and the TR had a total establishment of 209,537 men. Reserve brigades for the Machine Gun Corps were introduced in 1918.^{[12][11][13][14]} There was no change to the status or organisation of the SR battalions, but on 1 September 1916, the 3rd Line TF battalions (referred to as reserve battalions since 1 April) were amalgamated into only one, two or at most three per infantry regiment, and grouped into new reserve brigades bearing the titles of the TF divisions they were reinforcing.^[15]

The scheme was further developed to provide for the progressive training of recruits under 19 years old (when they could be sent overseas). Of the 112 TR battalions, 42 were selected: 14 became 'Young Soldier Battalions' and 28 became 'Graduated Battalions'. When a recruit had finished initial training in the Young Soldier Battalion he was sent to one of two associated Graduated battalions, in which the four companies were organised by age, for training in 3-monthly steps between 18 and 19 years. As a result, every 3 months, 28 companies of newly trained 19-year-old soldiers were ready for drafting to France. In due course the War Office decided that Graduated battalions could be used for home defence while the men completed their training (as was the case with the SR). The Graduated battalions then began to be transferred from the Reserve brigades to replace 2nd Line TF battalions in the Home Service divisions. They were renumbered from 201st upwards, but later were once again affiliated with a particular regiment and numbered as the 51st (G), 52nd (G) or 53rd (YS) battalions of their regiment.^[13][14][16]

• British Army during World War I
• British infantry brigades of the First World War