The job of the female secret agent, often disguised as a journalist, was to see if other agents could keep their mouths shut.
The files, released 70 years after the war, show that all too often Agent Fifi, as she was known, was able to get them to "spill the beans".
The agent, real name Marie Chilver, was employed to test out trainee agents in Britain before they were sent to occupied Europe in World War II, the BBC reported.
Suspicious that she was a German agent - she apparently looked too healthy for someone who had been in a prison camp, from which she had escaped - he called her "one of the most expert liars in the world".
However, once confident of her identity, the SOE, Britain's World War II sabotage organisation, put her to work in the UK masquerading as a French freelance journalist.
Most of them had not - one young promising Belgian agent was a case in point.
"Fifi" reported that by the end of the evening she had found out just about all there was to know about him, and his employment was terminated shortly after.
She insisted there was "absolute fairness" about her methods, saying it would help those trainees who did get through to "outwit all the Fifis they are likely to meet in their future career".
"Compared to what is most likely to happen in the field, it is very mild and innocent. It would be a pity to have to give up this method, because it does give the students a good chance of using their brains (or just their low cunning)," she wrote.
They have previously only been available in the reading rooms of the National Archives at Kew, it said.
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