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The original caption for this photo reads: 'Official photograph taken on the British Western Front. Highland gas sentry reading a letter from home. Note the gas alarm near at hand.'
Letters to and from home played an important part in maintaining morale, and it has been estimated that up to 12 million letters a week were sent to soldiers, many at the front line.
This photo is believed to have been taken by John Warwick Brooke, of the Topical Press Agency. Brooke was the second British Official War Photographer to go to the Western Front in 1916. He took what are regarded as some of the most memorable images of the First World War.
Official war photos were included in our free exhibition 'Behind the lines: Personal stories of the First World War', which ran from 27 June to 11 November 2014.