The lone MI5 agent who secretly penetrated the ranks of Britain’s wartime Nazi sympathisers has been identified as an unassuming suburban bank clerk.

Files released by the National Archives in Kew, west London, suggest the agent who posed as an undercover Gestapo officer was actually Eric Roberts, a father of two in his thirties who had been living quietly near Epsom racecourse in Surrey.

The disclosure comes after documents released earlier this year revealed how the agent – operating under the alias Jack King – had been able effectively to control the activities of hundreds of “Fifth Columnists”, neutralising the threat to Britain’s war effort.

The release 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.

From the brief details about him contained in the files, he seems an unlikely candidate for such clandestine – and potentially dangerous – work.

Even his colleagues working at the Euston Road branch of the Westminster Bank in Hampstead, north London, were taken aback when, in 1940, he was suddenly plucked by MI5 from obscurity for special wartime work of “national importance”.

The bank’s assistant controller, RW Jones, wrote to MI5, seeking an explanation.

“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 military importance which would take him away from his normal military call-up in October?”

According to his personal record sheet, his qualifications for performing the role of an undercover Gestapo officer appear thin as he had only a “slight” knowledge of German, although he had twice visited the country on holidays in the 1930s.

He had, however, somehow acquired a knowledge of the pro-Nazi groups operating in Britain in the period before the outbreak of the Second World War – although how he came to do so is not disclosed in the documents.

He was also a ju jitsu enthusiast – having been a member of the Anglo Japanese Judo Club – which may have proved handy for a secret agent if he found himself in a tight corner.

Whatever the reasons, Maxwell Knight – MI5’s top agent runner – was keen that he should be taken on.

“Roberts is thoroughly familiar with everything connected with the various pro-Nazi organisations in this country and Maxwell Knight has the highest opinion of his character and abilities,” one MI5 officer wrote.

“I should be very grateful if steps could be taken immediately to procure Roberts’ transfer from the bank to this office.

“Roberts has stated that he will be called up in October and the bank would in that case be paying the difference between his army pay and his normal salary.”

The scale of the support for Hitler uncovered by the “Jack King” operation – with the numbers running “certainly to scores and probably to hundreds” – shocked Britain’s wartime establishment.

In a report from September 1944, one of King’s handlers, TM Shelford, from MI5’s F3 counter-subversion section, said that in many cases it was down to a virulent anti-Semitism.

“There is no doubt that the information which we obtain from the Fifth Column organisation is, on the whole, accurate,” he wrote.

“It may be astonishing to some people who are not so familiar with the Fascist mentality as those of us who work in F3 that anybody could be as outspokenly disloyal as the persons about whom we get reports from Jack.

“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.”

 © Provided by Press Association Files released by the National Archives in Kew suggest an agent who posed as an undercover Gestapo officer was actually Eric Roberts, a father of two in his thirties (The National Archives/PA) The files contain numerous accounts of King’s meetings with Nazi sympathisers, including one from April 1943 with three women who were discussing a recent German air raid.

“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,” he wrote.

“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.”