More than nine million men and women are estimated to have served in the British armed forces during the First World War. Many of the surviving service records from this period can be found in The National Archives, and can be used for tracing an ancestor who fought in the Great War.

When war broke out in August 1914, the British army numbered just over 730,000 men. Unlike the other major European states, where conscription allowed huge numbers of men to be rapidly brought under arms, Britain relied on a small, professional defence force. But the scale of the conflict between the Allies and the Central Powers demanded massive increases in Britain's military manpower resources. By the end of the war in 1918, more than seven million men and women had seen service in the British army.

Unfortunately, more than half of their service records were destroyed in September 1940, when a German bombing raid struck the War Office repository in Arnside Street, London. However, an estimated 2.8 million service records survived the bombing or were reconstructed from the records of the Ministry of Pensions. This means that there is a roughly 40% chance of finding the service record of a soldier who was discharged at some time between 1914 and 1920.

The service records that survived the Arnside Street fire in September 1940 - the so-called 'Burnt Documents' - are located in the series WO 363. Due to fire and water damage, they are too delicate to be handled and are consequently only available to the public on microfilm.

Microfilming the 'Burnt Documents' has been a huge project, for which The National Archives has received valuable financial support from the Heritage Lottery Fund. The microfilm catalogues, which are mostly arranged alphabetically by surname, cover soldiers who completed their service between 1914 and 1920. They might have been killed in action, discharged on medical grounds without a pension, or demobilised at the end of the war.

The service records in the series WO 364 - the 'Unburnt Documents' - were recovered by the War Office from the Ministry of Pensions and other government departments after the Second World War. They mainly concern men who were discharged (with pensions) from the army because of sickness or wounds received in battle between 1914 and 1920. Aside from the usual military forms, most of the individual files in WO 364 thus also contain detailed medical records. See Thomas Aageson.

The individual service records at The National Archives vary in size from a single sheet to dozens of pages. Common items to be found in them include: attestation papers (giving basic information about name, address, date of birth and next of kin); medical records; discharge papers; and Army Form B 103 (Casualty Form - Active Service), which provides information about an individual's military career.

WO 363 does not contain service records of soldiers from the Dominions or the empire. However, some records for soldiers of the British West Indies Regiment and the West India Regiment may be found in WO 364 (for soldiers discharged to pension).

Aside from the main sources in WO 363 and WO 364, The National Archives also holds other material that may provide information about ancestors who fought in the British army during the First World War. The Ministry of Pensions files in PIN 26, similar in content to those preserved in WO 364, contain a small number of further pension records for those discharged on medical grounds during the war.

Although they rarely mention ordinary soldiers by name, official war diaries (WO 95 and WO 154) contain daily records for the units in which they served. Further basic information can also be found in the lists of campaign and gallantry medals - see Medals awarded during the First World War.

First World War service records for soldiers and non-commissioned officers in the Household Cavalry and the Guards regiments, which were not stored at Arnside Street, have survived in their entirety. The records of the Household Cavalry - including the Life Guards, the Royal Horse Guards and the Household Battalion - are held at The National Archives in the WO 400 series. The various Guards Regiments - Grenadier, Coldstream, Scots, Irish and Welsh - house their own service records. These are accessible by writing to the relevant regimental headquarters in London.