The division was mobilised on the outbreak of the Great War in early August 1914 and, when asked to serve overseas (as, according to the Territorial and Reserve Forces Act 1907, Territorial soldiers were not obliged to serve overseas), most of the men of the division volunteered. Those who didn't, together with the many recruits, were formed into 2nd Line battalions, the 2/1st London Brigade, part of 2/1st London Division, which later became 58th (2/1st London) Division. The battalions adopted the prefix '1/' (1/4th Londons, for example) to distinguish them from the 2nd Line battalions, which adopted the '2/' prefix (2/4th Londons).