10/28/2014 05:00 EDT | Updated 12/28/2014 05:59 EST

Eric Roberts, MI-5 spy, lived quiet life on Salt Spring Island, B.C.

In a tale that wouldn't look out of place in a John le Carré novel, an amateur historian who wrote a regular column for his local newspaper on Salt Spring Island, B.C., has been revealed as a key British spy during the Second World War.

Eric Roberts retired to the sleepy Gulf Island in the 1950s, staying mum on his impressive MI-5 career for the rest of his life. When he died in December 1972, aged 65, his obituary in Salt Spring's Driftwood paper noted he was a "free man of the city of London", and that he was the author of Salt Spring Saga, a history of the island he had come to call home.

And that seemed enough, until files declassified last week by the British National Archives identified Roberts, not just as a former spy, but as a pivotal agent in the infiltration of Hitler's "fifth column" who identified scores, if not hundreds, of Nazi sympathizers on U.K. soil.

A double life

Roberts, apparently a humble bank clerk for many years in London, was actually a double agent named "Jack King", who ingratiated himself among British supporters of Hitler by posing as a Gestapo officer, having been handpicked for the role by Maxwell Knight, then MI-5's top agent runner.

Amusingly, when Westminster Bank was approached in 1940 to release Roberts for work with the secret service, his employer agreed, but wrote back, "what we want to know here is, what are the particular and special qualifications of Mr. Roberts, which we have not been able to perceive."

In fact, Roberts was both ingenious and brave in his work, diverting the subversive activities of Nazi supporters into harmless efforts. According to the files, he also made sure that key intelligence data was rerouted to MI-5 rather than Berlin by offering to transmit secrets to Germany himself.

His work meant that secrets around significant strategies such as Operation Window — a tactic that employed strips of aluminum foil to jam Germany's radar during major raids, including the bombing of Hamburg — and the development of jet aircraft, never made it into German hands.

By 1945, Roberts' circle of reporting was considered the "most valuable single source of information" on British fascists and their activities.

Roberts moved to Canada with his wife Audrey and their three children, Maxwell, Peter and Crista.

Now 72 and living on Vancouver Island, Roberts' daughter, Crista McDonald, told CBC News that as a child she knew her father lived a double life and that his real job was working for MI-5.

A very charming man

"It's not a normal upbringing," she said. "You live knowing you don't draw attention to yourself."

Nevertheless, she said, she and her brothers had no idea of the scope of what their father did, or that he was secretly credited as one of MI-5's most valuable wartime agents. The declassifying of his file was a revelation.

"It meant a lot," she said. "I'm very happy — and we are as a family — that he's receiving some recognition. Even though he's long gone, it makes us very happy. We're finding it very emotional.

"He was a very charming, witty, unassuming man. Very loyal to his country. He was a great father."