To all appearances, Eric Roberts was a just ordinary man raising a family in wartime Britain.
He worked as a bank clerk in the Hampstead branch of Westminster Bank, had two children and lived a quiet life near Epsom racecourse in Surrey.
So it was a shock for his colleagues at the bank when, in 1940, they received notice from MI5 that Mr Roberts had been selected for special wartime work of "national importance".
Posing as a Nazi sympathiser and operating under the alias 'Jack King', he would go on to infiltrate the ranks of Britain's so-called 'Fifth Columnists', preventing an inside threat to Britain's war effort.
Mr Roberts' true identity has been uncloaked in documents released today by the National Archives in Kew, west London.
They detail numerous covert meetings Mr Roberts held with Nazi sympathisers, including one from April 1943 in which he describes three women's glee after an air raid on Britain:
A nearby school clinic was hit and Nancy Brown said with a grin that one expectant mother was killed, two girls badly injured, a clerk and two children killed. I looked in vain at the faces of these three women for signs of contrition. Nancy Brown looked a fine, healthy specimen of an Englishwoman, but it was obvious the deaths of these people meant absolutely nothing to her.
Mr Roberts found that many of those he came into contact with were driven to the Nazis by intense feelings of antisemitism. One of his handlers at MI5, T.M. Shelford, wrote in one of the documents:
Many people who were never members of the Fascist parties have been actuated by their anti-Semitic feelings to express the opinion that a German victory would be preferable to a British victory, since the latter would mean a victory for the Jews.
While Mr Roberts' suitability for such a secretive and important role remained unclear to his colleagues - one bank manager writing that he had "not been able to perceive" any "particular and especial qualifications" - MI5 was clear that he was the right man for the job.
Roberts is thoroughly familiar with everything connected with the various pro-Nazi organisations in this country and Maxwell Knight [MI5's top agent runner] has the highest opinion of his character and abilities.
It appears that as well as acquiring a thorough knowledge of Nazi groups, Mr Roberts was a martial arts enthusiast, having been a member of the Anglo Japanese Judo Club.
Today's release of documents sparked a flurry of speculation as to King's real identity, but Professor Christopher Andrew, the author of the official history of MI5, said it was now clear from the files that it was Roberts.
"Files in the latest release reveal for the first time that King's true identity was Eric Roberts," he said.