Hans Kohout had a plan, according to secret documents released Friday.

At the height of World War II, the naturalized British citizen 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. He knew about it from his work at a plant doing defense-related work.

Kohout passed the strategic 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, so they could copy the technology and put the weapon to use themselves.

But King was an impostor actually working for British intelligence, not the Gestapo, and Kohout's treasonous plan fizzled, according to the secret intelligence files made public by the National Archives. The information never crossed the English Channel.

Time and time again, the low-key "Jack King" was able to convince British traitors that he was a Gestapo man, hoovering 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 without special training.

Twigge said King's work helped to defuse 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. "He managed to flush them out and put a brake on their activities."

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 even 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 British soldiers and 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.

He did display typical British reserve: Field reports show Roberts was able to contain his utter disgust for those Brits willing to provide information that could easily have led to more carnage on the home front.

In a 1942 report, he described meeting a woman identified as Nancy Brown in the Brighton area along England's vulnerable southern coast. He said her friendly manner made it "almost impossible" for him to believe at first that she would betray her country — but within an hour she was giving him sensitive military data.

"The fact that the items of information volunteered might have resulted in the deaths of many people counted for nothing," he wrote.

The material Roberts gathered was never given to police for possible prosecution, and Kohout and Brown were never charged with any crime. The information was used by the intelligence service to keep track of active Nazi sympathizers in Britain during the war.