Hundreds more Britons went to fight fascism in the Spanish Civil War in the 1930s than had previously been thought, newly released files show.
MI5 recorded the names of about 4,000 people from Britain and Ireland suspected of travelling to join the war, National Archives files show.
The previous estimate stood at about 2,500. Many volunteers were communists and of interest to MI5.
One name on the list is Eric Blair, better known as author George Orwell.
His experiences in the Spanish Civil War were documented in his book Homage to Catalonia.
The details of those who had joined the fight against General Franco's forces between 1936 and 1939 continued to be updated by security service MI5 up until the mid-1950s.
The record for Orwell covers the period in which he published the bestselling novels Animal Farm and Nineteen Eighty-Four, until his death in January 1950.
The files, which can be downloaded free for a month, comprise more than 200 pages detailing the movements of the men and women who left British ports for the Spanish front line - as well as a "roll of honour" of some of those killed in action.
James Cronan, the National Archives' diplomatic and colonial records specialist, said it was not clear how many of those who left actually reached Spain, but he added that "we know that hundreds never returned".
"The International Brigades and associated militia brought volunteers together from all over the world in defence of democracy but few, if any, records exist of their service," he said. "That's why uncovering a document like this is so exciting."
This year marks the 75th anniversary of the start of the war in July 1936.