An estimated 4,000 Britons went to Spain during the Civil War, according to a detailed MI5 census. Many of them such as the writer George Orwell were monitored long after the conflict ended.
Among the newly-seen historical documents is intelligence gathered on artist and illustrator Paul Hogarth and communist George Ives, who both made moves to fight alongside Republicans in their war against Francisco Franco's nationalists, which took place between 1936 and 1939.
According to the files, Hogarth went to France in 1938 “with the intention of serving with the International Brigade” of foreign Republican sympathisers.
“But having enrolled found that he could not stand the rigour and hardship of war and eventually returned to England,” added the document, produced by the domestic MI5 intelligence agency.
Ives also encountered difficulties, according to the files.
“He is said to have gone to Spain in February 1938 and to have been captured in March of the same year,” they revealed.
He spent 10 months in Franco's jails, and returned to England in February 1939.
The issue of British nationals fighting foreign wars has hit the headlines recently with around 700 believed to be fighting alongside jihadist Islamic State fighters in Syria and Iraq.
Security services are concerned that they will return home with increasingly radical views, much as authorities feared that republican fighters in the Spanish Civil War would come back with deepened left-wing sympathies and open to advances from Soviet Union intelligence operatives.
“A number of former Brigade members figured in Cold War counter-espionage investigations, such as the American Morris Cohen, who later became a hero in Russia,” Cambridge University professor Christopher Andrew told AFP.
But experts warned against drawing too close a parallel.
“There is no similarity between International Brigades volunteers who went to fight in Spain and the fundamentalist Islamic jihadists now waging war in Syria and Iraq,” said Richard Baxell, author of “Unlikely Warriors”, a book about the British International Brigade volunteers.
“But it's true that both faced a hostile response from the British government, especially those who wanted to return to the UK. It is an interesting parallel,” he said on his blog.