General John de Chastelain's mother, who helped run an anti-Vichy spy in the Second World War

MARION DE CHASTELAIN, who has died in Alberta aged 89, was the American mother of General John de Chastelain, the monitor of IRA arms decommissioning; during the Second World War she had worked for Sir William Stephenson, the Canadian then running a spy ring for Britain in New York.

As a member of Stephenson's inner office at the British Security Co-ordination operation in the Rockefeller Center, she was controller for an American agent, Cynthia, who was sleeping with the Vichy French embassy's press attaché in Washington. At least once a week Marion de Chastelain would fly to Washington, take a taxi to the centre of town, and walk the last two blocks to the hotel, which she would enter through a side door for their rendezvous.

Cynthia, a tall, blonde socialite who revelled in the excitement, would hand over a package which Marion placed in her specially large handbag. It contained the press attaché's notes of briefings, conversations and telegrams.

Marion de Chastelain carried the telegrams back to New York, where she would translate them. She then returned with them the same day before they were missed from the embassy safe.
This enabled the Allies to read traffic between a French admiral on Martinique and several French warships; it is thought to have played a part in breaking the Vichy naval code before the Torch invasion of North Africa in 1942.

Although the two women, who knew each other only by their Christian names, got on well during their brief meetings, Marion de Chastelain regularly found herself having to refuse Cynthia's request for a meeting with the mysterious Stephenson.

Apart from her liaison work, Marion de Chastelain acted in Stephenson's office as a cypher clerk. Her British husband, Gardyne de Chastelain, an officer of the Special Operations Executive, visited North America in search of recruits. But when the centre of the Allied war effort switched to Britain, she returned to London with their two small children.

Soon afterwards her husband was captured in Romania and imprisoned for months by its pro-Axis government, though he was kept out of German hands. Marion de Chastelain went to work for Department 5 of the Secret Intelligence Service, processing intercepted messages with the aid of her fluent French and German and passable Romanian.

With the return of peace she declined an invitation to continue with Intelligence work, choosing to concentrate on her family while her husband, who had been awarded the DSO and OBE, set up a London-based export company.

She had been born Elizabeth Walsh on May 24 1910 at Freehold, New Jersey, the daughter of a Standard Oil executive. When she was nine her father was posted to the Romanian oil fields, and she was sent to school in Switzerland before studying international law at the Sorbonne in Paris. In 1933 Marion married Alfred Gardyne de Chastelain, a Scotsman working for British Petroleum in Bucharest. On the outbreak of war in Western Europe she took their two children, 18-month old John and three-year-old Jacqueline, to London, then went back to Romania.

But after her husband went to Istanbul and Cairo for SOE, shortly before Romania joined the Axis, she returned to England brandishing her American passport, and then took the children to stay with her parents in New York. One day the telephone rang, and a voice asked if she would care to do something for King and Country. "Why not?" she replied.

On the return of peace, Gardyne de Chastelain left SOE, and the family remained in London, where their house happened to overlook the back of the Russian embassy. He then went back into the oil business, becoming vice-president of Eastman International in Calgary. Later the de Chastelains moved to Australia, where for 10 years he was an adviser to the Bank of New South Wales. After that they returned to Alberta, where he died in 1974.

Marion de Chastelain spent 13 years as an executive assistant to the president of Westburne Petroleum Services in Calgary. She kept in touch with Stephenson, and visited him in New York and Bermuda every year. She recalled that she was always able to stare him down.

But although for a long time she declined to say much about her wartime work to her family, she agreed in 1980 to some discreet questioning about Stephenson on the Toronto television programme Front Page Challenge. This was on the occasion of the Tory Government awarding Stephenson the Order of Canada.

Increasingly angry at the way historians were emphasising the slenderness of the evidence about Stephenson, Marion de Chastelain amazed her son by giving a frank interview to a Winnipeg journalist, Bill Macdonald, in exchange for information about Stephenson's obscure origins in the prairie city.

But she never exaggerated what she had done during the war. "It was just a job," she said.