How a triple agent called 'the Cat' got the cream of Britain's spy network

Papers released by the Public Record Office reveal panic over a femme fatale and a mole in the Prime Minister's family

He was the new head of Britain's secret wartime department responsible for running spies behind enemy lines. She was "the Cat", a seductive French triple agent notorious for changing sides as often as she changed lovers.

When Lord Selbourne, chief of the Special Operations Executive (SOE), looked into the green eyes of Mathilde Carré at a cocktail party at Claridge's in 1942, he felt the power of the spell that had bewitched members of the Gestapo and the French Resistance alike.

Carré was determined to become his mistress, and Selbourne appeared ensnared, asking her to dinner, promising to commend her to Winston Churchill and assuring the Parisienne with a taste for luxury that she would have everything she needed in London.

His ardour caused deep consternation among Britain's spymasters, who feared he was about to "make a fool of himself" with one of the most dangerous spies of the war.

MI5 documents declassified today reveal serious concern that Selbourne was embarking on an affair with an "extremely dangerous" woman who had the potential to betray Britain's entire overseas spy network to the Germans.

Carré had been second-in-command in the French Resistance organisation Interallié, where she helped provide the Allies with virtually the complete shape of the German military in France, winning over Gestapo officers with her jet black hair, voluptuous figure and long shapely legs.

Interallié was exposed by German agents in November 1941 and she began working for the Nazis after being given a meal in one of Paris's best restaurants and offered a hotel suite and a 60,000-franc monthly salary. She enthusiastically began betraying former comrades, became the lover of the German agent who had won her over, and continued sending messages to London as if Interallié was still functioning. Carré, then 34, travelled to England in February 1942 with her main lover, the French Resistance leader Pierre de Vomecourt, under instruction to expose the workings of the SOE. After arriving in London she admitted working for the Germans but agreed to change sides again and help the Allies by sending false information about SOE's activities to her Nazi contacts in France.

MI5 and the SOE remained deeply suspicious, however, and were alarmed when Carré, whom they codenamed Victoire, charmed Selbourne, who had just taken over as head of SOE, at Claridge's on 2 May.

An SOE informer, known only as Mrs Barker, who was living with Carré, said Selbourne had asked her out to dinner, had offered to mention her to Churchill and to get her "everything she needed".

The message, which is contained in the papers released by the Public Record Office, said: "Victoire seems to be dreaming of becoming Lord Selbourne's mistress. According to her, he has all the attributes she admires in a man except that he cannot dance.

"Lord Selbourne may be merely playing up to her but even if only half of what she has told me is true it seems that he is behaving exceedingly foolishly and is not doing himself any good nor, for that matter, us."

The next day Captain Christopher Harmer, from the SOE, said it was unlikely that Selbourne would know Carré's full history and suggested sending an agent to ask him "what the position was" between them. "From the point of view of running the case I don't much mind whether she goes on seeing Selbourne or not but whether we owe a duty to him to prevent him making a fool of himself is a matter which I must leave for someone else to decide," he said.

As concern grew, Mrs Barker warned Captain Harmer that Carré was an "utterly egotistical woman who cares for nothing and nobody but herself" and would burst into fury at any opposition.

"Given a chance, she would sell any information she has to the other side. Added to all this, there is, of course, her interest in men. She feels she is irresistible to men anyhow and to sleep with a man seems a necessity to her. Once she gets hold of a man it is up to her to drop him and be unfaithful to him and God help the man or for that matter the service he is in, if he dares to drop her.

"I think she is an exceedingly dangerous woman ... war is merely a means to an end for her, viz her amusement ... and a life of luxury."

The papers do not record if Carré was successful in seducing Selbourne, but historians believe he would have been warned off quickly because the risk of associating with her was so great.

Russell Miller, author of Behind the Lines, a history of the SOE, said the prospect of an affair would have caused "panic" in Britain's secret services.

He said: "Mathilde Carré would probably rank alongside Mata Hari in many ways. She was a significant traitress and a number of SOE agents encountered her at their cost. Many were arrested and interrogated in a brutal way by the Gestapo.

"The prospect of Selbourne having an affair with her would have caused substantial panic in high quarters in the security service."

Oliver Hoare, an MI5 historian based at the Public Record Office, said there was no record of Selbourne being warned off but it would probably have been done orally and not recorded.

M R D Foot, the leading expert on the history of the SOE, said although Carré might have dreamt of being Selbourne's mistress, there was very little chance she was successful.

"Selbourne may have been polite to her because she was at that stage being handled with kid gloves because it was hoped that some use could be made of her. It was not yet clear that her allegiance was more to the Germans than the Allies.

"The fact that she dreamt of capturing him from Lady Selbourne is one thing but I think the idea that she became his mistress can be ruled out straight away.

"Selbourne knew how a gentleman should behave. He was also too busy running the SOE and the Ministry of Economic Warfare to have time to have a mistress."

Carré was arrested in July when British investigations discovered the extent of her betrayal of Interallié. She was interned at Aylesbury prison in Buckinghamshire until 1945 when she was returned to France where she was tried for "giving intelligence to the enemy" and sentenced to death. The sentence was commuted twice and she was released in 1954. She is thought to have died in 1971.

Her dalliance with Selbourne did not appear to result in the loss of any secrets. Captain Harmer recorded in a later summary that "observation was kept on Victoire throughout her stay and it is thought impossible that there was any leakage to the Germans".