An extremely effective MI5 World War Two operative who pretended to be in the Gestapo and befriended British Nazi sympathisers was a quiet bank worker from Epsom, it has emerged.

Files released by the National Archives in Kew last week show that the double-agent who worked under the pseudonym Jack King, was actually Eric Arthur Roberts, who was in his 30s when he was recruited by MI5.

Mr Roberts, of Tattenham Grove, Tattenham Corner, was a father-of-two who worked at the Westminster Bank in Hampstead Road, London.

He had worked at the bank for 15 years before he was recruited to MI5 by senior agent Maxwell Knight who had "the highest opinion of his character and abilities".

The files revealed that Mr Roberts was "thoroughly familiar with everything connected with the various pro-Nazi organisations in this country".

R.W. Jones, his bank’s assistant controller, wrote to MI5: "What we would like to know here is what are the particular and especial qualifications of Mr Roberts which we have not been able to perceive - for some particular work of national importance...?"

Roberts’ role involved him posing as a Gestapo officer, acting as a focus for information gathered about the war effort by Nazi sympathisers in Britain - ensuring it did not reach the enemy.

He duped Nazi sympathisers into believing they were spying for Hitler and even handed his ‘spies’ fake German medals, made by MI5, in recognition of their work for The Third Reich.

Some of the Nazi sympathisers were ordinary people who were motivated by violent anti-Semitism, some were expatriate Germans, while others were members of Sir Oswald Mosley’s British Union of Fascists.

The files released contain material consisting of transcribed conversations between Jack King and members of his pro-Nazi group, bringing in reports about the Allied war effort suitable for passing to Berlin.

The subjects discussed include the need for a renewed Blitz, with more bomb damage aimed at undermining public morale, and possible aid to an invading German force.

Mr Roberts was born in Sussex and spoke Spanish and French and some Italian, Portuguese and German. He was a member of the former Anglo Japanese Judo Club.

After the war Mr Roberts left the M15 and moved with his family to Canada, where he died in 1972.