Eric Roberts

LONDON — At the height of World War II, Hans Kohout wanted to give the Nazis advance word of a top-secret British tactic that could neutralize an enemy’s air defenses, leaving major cities exposed to devastating air raids. Kohout, a naturalized British citizen, knew about it from his work at a plant doing defense-related work.

He passed the information to Jack King, who he believed was a Gestapo agent working undercover in Britain. Kohout expected King to give the information to the Nazis.

But King was an impostor working for British intelligence, not the Gestapo, and Kohout’s treasonous plan fizzled, according to the secret intelligence files made public Friday by the National Archives.

Time and time again, the low-key “Jack King” was able to convince British traitors that he was a Gestapo man, gathering up potentially lethal information intended for the Nazis.

“It was a brave undertaking, mixing with fascists, pretending to be someone you weren’t. It was dangerous work that could have gone wrong,” said Stephen Twigge, a historian with the National Archives, whose documents revealed that King was actually Eric Arthur Roberts, a bank clerk.

Twigge said King’s work helped to neutralize a potential “Fifth Column” that might have damaged Britain’s war effort. The files suggest the number of Nazi sympathizers willing to take action against British forces was larger than had been thought, he said.

“He was infiltrating a network, putting himself forward as the middle man in German intelligence,” said Twigge.

One of Roberts’ handlers, identified only as T.M. Shelford, said many of the Nazi sympathizers in Britain were motivated by a dislike for Jews.

“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,” he wrote in 1944.

The documents show Roberts had a solid if unspectacular career at Westminster Bank when Security Services asked for him in 1940. His boss sounded surprised by the high priority placed on his services: “What are the particular and especial qualifications of Mr. Roberts — which we have not been able to perceive — for some particular work of national importance?” his supervisor wrote.

Roberts vanished for a time, re-emerging as Jack King, who had as one of his duties gently discouraging his contacts from sabotaging military installations.

The files reveal little about Roberts’ methods besides proficiency at several languages. But he had only rudimentary German and visited the country only twice before the war.

The material Roberts gathered was never given to police, and Kohout was never charged with any crime. The information was used to keep track of active Nazi sympathizers.

Intelligence files do not reveal what Roberts did after the war.