He is known to the world as the author of bestselling children’s books such as Charlie and the Chocolate Factory and James and the Giant Peach. Yet before he became a successful writer, Roald Dahl had a very different reputation – as the sexiest British spy in America.

A ribald portrait of Dahl’s second world war years as an undercover agent attached to the British embassy in Washington emerges from the pages of a new biography that credits the writer with a very special talent for the Anglo-American special relationship.

“Girls just fell at Roald’s feet,” declares Antoinette Marsh Haskell, the daughter of Dahl’s closest American friend. “I think he slept with everybody on the east and west coasts that [was worth] more than $50,000 a year.”

Drawing on a previously unpublished trove of Dahl letters and other documents, Jennet Conant, an American journalist, has written what may prove the most comprehensive account of Dahl’s raucous wartime exploits as a charming RAF attaché.

His conquests included Millicent Rogers, the glamorous heiress to a Standard Oil fortune; and Clare Boothe Luce, a right-wing congresswoman and the sexually frisky wife of the publisher of Time magazine.

Dahl would later complain to friends that Boothe Luce, 13 years his senior, had left him “all f***** out” after three nights of bedroom capers.

“Dahl’s superiors watched his rake’s progress with grudging admiration,” Conant writes in The Irregulars, to be published in Britain on September 9. “A certain amount of hanky-panky was condoned, especially when it was for a good cause.”

Injured during training as an RAF pilot, Dahl fought in the Middle East before he was declared unfit to fly and was shipped to the Washington embassy in 1942. He immediately cut a swathe as a 6ft 6in battle-scarred pilot who was nonetheless horrified to find himself “in the middle of a cocktail mob in America”.

He struck up a friendship with Charles Marsh, a self-made Texan newspaper magnate who was a fan of Winston Churchill and a ready ally in the British effort to win American support against Adolf Hitler. It was Marsh’s family that provided Conant with a large cache of letters that the two men wrote to each other during the war years.

“We sort of adopted him,” Antoinette told Conant. “Roald was a real charmer when he wanted to be. He was great fun to be around, he was always doing tricks and playing crazy practical jokes.”

With Marsh’s help, Dahl became close to prominent American journalists and senior US officials, notably Henry Wallace, the isolationist vice-president. His social-climbing skills attracted the attention of William Stephenson, the Canadian spymaster, who was running a clandestine British effort to draw America into the war.

Much has already been written about the activities of Stephenson’s then secret British Security Coordination (BSC) outfit, which included Ian Fleming and Noël Coward among its high society agents. Conant, though, focuses on Dahl’s minor but colourful role as the languid seducer who was eventually befriended by Eleanor Roosevelt, wife of President Franklin D Roosevelt, and who became a regular visitor to the White House.

Dahl leapt at the chance to join the world of codenames and secret passwords, which added at least a veneer of significance to his otherwise dreary daily round of drinks parties, banquets and balls. His job, writes Conant, was “to be as engaging as possible, a bright and breezy presence at table, and encourage confidences from those in the know”.

The Marshes were left in little doubt about the fringe benefits of Dahl’s diplomacy. “There was a parade of women,” said Antoinette. At one point he turned up at the Marshes’ home with a woman who was said to be General Dwight Eisenhower’s mistress. “I think he liked to show them off to my father,” Antoinette added.

A previous biography of Dahl described him as “one of the biggest cocksmen in America”, and Conant relates in gripping detail how he earned the accolade. At a British embassy dinner, Dahl was deliberately placed next to Boothe Luce, whose anticolonial tirades and distaste for Churchill were worrying British officials.

It was Dahl’s job to get close to Boothe Luce, which he managed only too easily. She proved such a tigress in bed that Dahl later claimed to have begged his superiors to take him off the assignment. He was ordered back to the bedroom, and told to close his eyes and think of England.

During the course of his kissing and telling career, Dahl managed to pass on several useful intelligence titbits and a couple of purloined documents. He came to believe from his visits to the Roosevelt family home in Hyde Park, New York state, that the crippled president was having an affair with Crown Princess Martha of Norway, who had been granted asylum by the US.

Perhaps more significantly, as things would turn out, Dahl also discovered a talent for writing. In April 1943, he published a children’s story called The Gremlins – about RAF pilots and their bad luck.

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