Details of Jewish soldiers who fought Franco's fascists during the Spanish Civil War are included in an extensive list of veterans revealed for the first time by the National Archives.

They were among around 4,000 British volunteers who joined the International Brigades and were later investigated by MI5 for breaching the British government's non-intervention policy.

Around a fifth of the volunteers were thought to have been Jewish. They defended the Spanish Second Republic after the 1936 military rebellion led by General Franco's nationalists and supported by Nazi Germany and Mussolini's Italy.

Sam Lesser, from Brixton, south London, was highlighted by the National Archives as one of the most high-profile fighters. He had been radicalised by the depression of the 1930s and the spectacle of Oswald Mosley's antisemitic blackshirts.

By October 1936 Mr Lesser was in a unit dispatched to Madrid to stop the nationalist advance. Wounded in action, he was one of only six to emerge alive and later began a career in journalism as a correspondent for the Daily Worker. He died in October last year at the age of 95.

His index card - compiled by MI5 - is one of the documents found in the archives and made public. Dated January 1953, it gives details of his name and address as well as describing him as being of "Jewish appearance".

Lou Kenton is also included on the list, which notes that he joined an ambulance unit, while Joseph Kahn was among a group which sailed to Dieppe with a convoy of lorries in December 1936. The trio were honoured with Spanish citizenship by the Spanish embassy in London two years ago.

A statement from the National Archives said the discovery of the list showed the number of volunteers who left Britain for Spain may have been far higher than previously thought. The file was first opened last August, but its contents have only been revealed for the first time this week.

MI5 investigated the British volunteers and suspected Communist Party members and compiled reports on those who defied the government ban to fight in Spain.

James Cronan, of the Archives, said: "The records show that the security service tracked the movements of around 4,000 people it believed were trying to travel to Spain to fight with the International Brigades, many more than previously thought. It's not clear how many made it to Spain although we know that hundreds never returned.

"The International Brigades brought volunteers together from all over the world in defence of democracy but few if any records exist of their service. That's why uncovering a document like this is so exciting."

The list is available to view online and includes entries for Eric Blair - better known as George Orwell - union leader Jack Jones, and poet John Cornford.

A ceremony will be held on Saturday July 2 at the International Brigades Memorial in Jubilee Gardens, London to mark the 75th anniversary of the start of the civil war. Survivor David Lomon (né Solomon) will lay a wreath on behalf of Ajex, the Association of Jewish Ex-Servicemen and Women.