Spy's love for her dog put D-Day secret at risk

Second World War documents reveal MI5's difficulties in handling demands of troublesome and temperamental double agents

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A double agent, instrumental in deceiving Germany about the location of the D-Day landings, threatened to betray the Allies on the eve of the invasion because she blamed the British for the death of her pet dog.

Nathalie Sergueiew, a Russian whose family fled to France during the 1917 revolution, had earlier threatened to stop working for MI5 because they would not transport her dog, Frisson, from Gibraltar to England. The threats by the temperamental Sergueiew – who operated under the code name Treasure – were part of a pattern of behaviour that so infuriated MI5 that the service sacked her once D-Day was over.

Born in Russia in 1912, Sergueiew was recruited by the German Secret Service – the Abwehr – in Paris in 1940, trained in espionage techniques then sent to Spain to work her way to spying in England, using the code name Tramp. However, she handed herself in to the US embassy in Madrid and volunteered to work for the Allies. She walked into the offices of Kenneth Benton, the MI5 head of station, carrying Frisson, and demanded, unsuccessfully, that her pet should be allowed to bypass quarantine laws and travel with her to England.

She took a boat from Gibraltar, leaving Frisson behind, and arrived in England in November 1943, where she was put to work feeding the Germans misinformation as part of Operation Fortitude – the campaign designed to convince Hitler that the Allies would land at Pas de Calais rather than Normandy.

MI5 files released by the Public Record Office yesterday show that within weeks the agent, later described as "exceptionally temperamental and troublesome", had started creating problems. In December Mary Sherer, the MI5 operative assigned to run Treasure, reported to Lieutenant-Colonel Tommy Robertson, one of the architects of the double-cross system, that she was "very upset about the absence of her dog".

An American boyfriend had let her down by not smuggling it into Britain and Miss Sherer asked if it was possible to get the Navy to help. "She has seriously threatened that if the dog does not arrive soon, she will not work any more," Miss Sherer said. "I think this can be dealt with but it will mean a scene. I do not quite see what we can do to help, because if we have the dog sent over here officially it will have to go into quarantine, which from Treasure's point of view would be as bad as having it in Gibraltar."

Later that month, Miss Sherer reported that Treasure had been behaving in an "eccentric manner", wandering round in her pyjamas, sitting on the drawing-room floor and dancing. By January she appeared to have calmed down, and Miss Sherer recorded that Treasure was resigned to Frisson remaining in Gibraltar.

Treasure caused alarm later that month when she admitted she had told a US Air Force officer, Lt Kenneth Larson – with whom she said she was in love – that she had been in touch with the Germans. Miss Sherer was concerned she would "have trouble" with Treasure because Larson's colleague, Capt Clifford Lord, had taken the dog to her sister's in Algeria but brought it back to Gibraltar rather than leaving it there.

Relations between MI5 and their agent worsened after a trip to Lisbon in March, to pick up a radio from her German spymaster, Emile Kliemann, caused problems with the Portuguese authorities. Some of her clothes went missing on the return journey and MI5 was reluctant to pay the bill. At about this time, Treasure also learnt that her dog had died.

On 17 May 1944, three weeks before D-Day, Miss Sherer reported that Treasure had confessed that her transmissions to the Germans included a secret signal, which she planned to leave out, letting the Nazis know her cover was blown. "She confessed that her motive was revenge for the death of her dog, for which she considered we were responsible," Miss Sherer said. MI5 did not believe Treasure's claims about a secret signal and considered imprisoning her, but decided not to act until after D-Day. She had been ill and went into hospital on 5 June– the day before the invasion. On 15 June, the day after she came out of hospital, Lt-Col Robertson sacked her and warned of "severe action" if she did anything to threaten the Allied cause.

By the end of the month she had offered to work for the Free French forces and had been accepted, but she remained a thorn in the side of the intelligence services, threatening to write a book about her spying experiences. Lt-Col Robertson went to great efforts to find out what was in her manuscript, concerned she would name officers involved in the Operation Fortitude and asking for FBI help when Treasure was rumoured to be moving to the US.

She did eventually write her memoirs, Secret Service Rendered, which was published in 1968. But in the event it was more concerned with personal relationships than with the Allied intelligence effort.

The files on Treasure are part of 200 documents made public in the largest release of security service documents. They are released to coincide with a new exhibition on espionage at the Public Record Office in Kew, which begins on Saturday.