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He was nothing like the spies you see in countless Hollywood films.

Eric Roberts worked as a humble bank clerk while secretly posing as Gestapo agent Jack King to infiltrate UK fascist groups during the Second World War.

The MI5 agent was so successful he gained the trust of 500 Nazi ­sympathisers and stopped vital information making its way to Hitler’s Germany.

But his family had no idea about the extent of his espionage until it was revealed in National Archive papers in 2012.

Granddaughter Marilyn said: “As a child I knew grandad was a spy during the war but our family had no idea what that really involved.

“He never talked about it and it was never spoken about, other than in ­whispers during sleepovers with my ­cousins.

"We would make up all these pretend ­stories about his adventures.”

Even her dad Max, 84, knew nothing about his ­father’s double life.

Marilyn, 45, of Toronto, Canada, said: “My dad remembers his father ­disappearing for days but he had no idea what he was up to.

"He also recalls going into London with his dad who insisted on walking five feet in front of him.

“When the phone rang, Dad was told to hand it over to his father, no matter what name the caller gave.”

Author Robert Hutton approached the family to tell Eric’s story and his book Agent Jack was released last year.

(Image: National Archives)

It has echoes in a new Amazon drama about Nazi hunters in America due to be shown this month.

In the series Hunters, Al Pacino leads a team bringing secret US Nazis to justice in the 1970s.

Britain’s real-life Nazi hunter was a far cry from Pacino’s tough guy image but no less daring.

(Image: ALEX LUKEY PHOTOGRAPHY)

Eric was recruited by MI5 spymaster Maxwell Knight – said to have inspired Bond character M – in the 1930s.

At that time fascism was on the march all over Europe and MI5 was worried about its popularity here.

Eric joined the British Union of Fascists, led by Oswald Mosley and got to know many of Hitler’s British sympathisers.

(Image: Bettmann Archive)

While working at Westminster Bank he posed as a Gestapo agent under orders to recruit people to Germany’s cause.

Hutton says: “People ­trusted Eric implicitly.

“He was fantastically ­outgoing, a real practical joker and very charming. His personality got him out of a lot of scrapes.”

For example in 1940 a gang of fascists in Leeds rumbled him when he gave a false job and ­address. But he managed to talk his way out of it.

Hutton said: “He wrote that they were ‘hostile’ but explained ‘I was able to set their mind at ease.’ I think they trusted him because they wanted to.

“He had that quality of being a nice guy. It’s not very James Bond but it’s the way he made spying work for him.”

(Image: Amazon Prime Video)

Eric took early retirement in 1956 aged 49 and moved with wife Audrey to Vancouver, Canada, with their two sons Max and Peter and daughter Crista.

Eric believed his past was behind him but MI5 got back in touch in 1968 after Kim Philby and others were exposed as double agents who passed information to the Soviet Union during the war.

Officers questioned him about his wartime ­activities and Eric ­was concerned that they suspected him too.

(Image: UGC)

The worry affected his health and he died four years later.

Marilyn said: “My dad said the stress of that visit really affected my grandfather.

“He had diabetes and after MI5 came knocking he went downhill.

“He was a really good human being and we are just so proud of him.”