Nathalie Sergueiew, a French woman with Russian extraction, codenamed Treasure, was one of MI5's most effective agents operating its wartime Double Cross, or XX, System.

At the outbreak of war, she was recruited by German intelligence, the Abwehr, through a journalist friend. She was sent to Madrid where she approached an American friend and offered to spy for the allies. The friend put her in touch with the British embassy there.

With her German controllers believing she was still loyal to them, she was brought to England by British agents. However, her pet dog, Frisson, was left behind in Gibraltar.

"Treasure is very upset about the absence of dog, and has seriously threatened that if the dog does not arrive soon she will not work any more. I think this can be dealt with but it will mean a scene," her MI5 case officer, Mary Sherer, warned in December 1943.

"I am afraid Treasure's Amercian boyfriend has let her down and has no intention of smuggling the dog over here for her. I am wondering whether we could get the Navy to help," Sherer wrote.

MI5 was also concerned that her cover might already have been blown by an affair with an American airman, Lieutenant Kenneth Larson, whom she had met in Gibraltar and had confided in about her intelligence work.

"She said she had been very stupid, but she had fallen in love with Larson and he with her, and for some very complicated reason of her own she had told him in order to see if he trusted her," Sherer noted.

Sergueiew learned that Frisson had died just as MI5 was preparing her for a crucial period in her role as double agent. On May 17 1944, less than a month before D day, she told her handlers she had agreed a secret call sign with Emile Kliemann, her Abwehr controller, so that he would know whether her transmissions were genuine.

"Further, she had meant... to get the W/T (wireless transmitter) working well then blow the case by omitting the signal," Sherer reported. "She confessed that her motive was revenge for the death of her dog for which she considered we were responsible".

Sergueiew eventually gave details of the coded call sign to Sherer. But A week after D-Day, Sherer's MI5 boss, Colonel TA Robertson, angrily confronted Sergueiew telling her her services were no longer required. He called her a "wretched woman".

Despite MI5's criticism, Government codebreakers monitoring her wireless traffic and the German response to them praised her saying she had "saved [their] bacon".

At the end of the war Sergueiew returned to France where she threatened to publish her memoirs. MI5, who tried to enlist the support of Malcolm Muggeridge, an Mi6 officer in France, failed to stop her.