Wartime records have revealed the existence of a female secret agent whose job was to see if other agents could keep their mouths shut.
The files 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 Two.
Her file is one of thousands released by the National Archives in London.
Chilver, the daughter of an English father and Latvian mother, first came to the attention of the Special Operations Executive (SOE) in 1941 when she helped an airman - Flt Lt Simpson, shot down over France - get back to England.
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 put her to work in the UK masquerading as a French freelance journalist.
Chilver would start chatting to trainee agents in hotels and bars to see if they had learned how to keep secrets.
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.
According to the National Archives blog, Chilver "was known to be stubborn, even stroppy - but with a strong moral sense".
She won compensation from the Soviet Union for property taken from her Latvian family, and used some of the money to set up an animal charity in the country.
She lived in the Wye Valley on the English-Welsh border with her lifelong friend - and fellow former intelligence officer - Jean Felgate.
Chilver died on 5 November 2007. Her file is among 3,300 intelligence and security documents from World War Two being made available for the first time online by the National Archives.
They have previously been available only in the reading rooms of the National Archives at Kew.