The unassuming bank clerk who infiltrated ranks of British Nazi sympathisers

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:

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:

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.

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.