So, did Winston Churchill have an affair with model Cara Delevingne's great aunt, Doris?
He led Britain through its 'Darkest Hour' and was admired for being steadfastly faithful to wife Clemmie, but now a documentary claims he made an exception for 'ecstatically beautiful' Lady Doris Castlerosse
He was to become the wartime hero who led Britain through its darkest hour, when it stood alone and defiant against Hitler - although he was also, according to one biographer, "probably the least dangerously sexed major politician since Pitt the Younger".
She, on the other hand, was "the most ecstatically beautiful woman in London", who maintained "there is no such thing as an impotent man, only an incompetent woman". And, it is now claimed, Doris, Lady Castlerosse, proved she was no incompetent by seducing the most seemingly impossible target of all: Winston Churchill himself.
There had been rumours before, of course. Now, though, a Channel 4 documentary, to be broadcast on Sunday, is to reveal the recorded revelation of one person who might have been expected to know the truth: Jock Colville, Churchill's private secretary.
In 1985, Colville unburdened himself, on tape, to the archivists at Churchill college, Cambridge, but not before insisting: "This is a somewhat scandalous story and therefore not to be handed out for a great many years."
"Winston Churchill," said Colville, who died in 1987, "was not a highly sexed man. I don't think that in his years (of) married life he ever slipped up, except on one occasion, by moonlight in the south of France ... He certainly had an affair, a brief affair with ... Lady Castlerosse, as I think she was called ... Doris Castlerosse, yes, that's right."
If true, this is nothing short of extraordinary. Previous historians have regarded Churchill as having been so devoted to his wife Clemmie (Clementine) that he never strayed from the woman he married in 1908. This, it is said, was the one husband who could resist the advances of Daisy 'Wanton' Fellowes when she offered herself to him lying naked on a tiger-skin rug in 1919.
But Lady Castlerosse, we are now led to believe, was made of even more extraordinary stuff. "She had only to raise an eyebrow," declared one admirer. And what eyebrows they were."
For, as every journalist who reports on Lady Castlerosse must delightedly affirm, she was none other than the great aunt of Cara Delevingne, the supermodel whose own eyebrows have helped make her famous.
Cara's grandmother Angela - "a noted beauty, who evaded a kidnapper in Harrods and declined a part in Gone with the Wind" according to her Daily Telegraph obituary - defied her mother to marry Lady Castlerosse's impecunious brother, Edward Dudley Delevingne.
It was 1933. Churchill was enduring his "wilderness years", before wartime duty and glory called. His wife, who did not care for the louche Riviera set, was not there. Lady Castlerosse was. He painted her three times, once when she was draped seductively over a chaise longue. And, according to historians Warren Dockter and Richard Toye, whose research underpins the documentary, Churchill did more than take an artist's interest in her "flower petal complexion and blue eyes with enormously long dark lashes" (to use the description of another admirer).
The academics are backed by Caroline Delevingne (Doris's niece, Cara's aunt), who, the documentary-makers say, has become the first in the family to give a televised interview about the affair.
"My mother (Angela) had many stories to tell about when they stayed in my aunt's house in Berkeley Square," Caroline tells the documentary. "When Winston was coming to visit her, the staff were all given the day off. Doris confided in my mother about it - they were good friends as well as being sisters-in-law - and so, yes, it was known that they were having an affair."
The historian Andrew Roberts, biographer of Churchill, said Jock Colville's taped allegation of Churchill's affair had not escaped the attention of previous historians.
"Having been researching a biography of Churchill for the past four years ... I listened to it (the tape) many months ago, as has Allen Packwood, the director of the Churchill Archives. And Colville actually said the words to another historian, Dr Correlli Barnett, back in 1985.
"When I heard the tape, I decided to investigate the allegation closely and found that the facts, and other correspondence in the archive, simply do not support it."
Churchill painting Lady Castlerosse thrice meant nothing. He painted Clemmie three times, says Roberts. "He also painted Sir Walter Sickert's wife Therese, Arthur Balfour's niece Blanche Dugdale, Sir John Lavery's wife Hazel, his own sister-in-law Lady Gwendoline Churchill, his secretary Cecily Gemmell, his wife's cousin Marryot White, and Lady Kitty Somerset. There is no suggestion he was sleeping with any of them."
Nor is the historian swayed by the testimony of Cara's aunt: "The fact that the Delevingne family, including the supermodel Cara and others who were also not alive at the time, claim that an affair took place is equally flimsy evidence.
"Plenty of people like to claim notorious links with the famous, as Cara herself must have discovered by now. Similarly, the sly insinuation that servants were given the evening off so that Churchill could have sex with Lady Castlerosse can be easily explained by the fact that they wanted privacy to talk and gossip.
"Servants were known to sell overheard information to newspapers. Even palace servants were not allowed into the weekly lunches that Churchill had with the king during the war."
Lady Castlerosse was born a haberdasher's daughter in London in 1900. Doris followed her dad into the clothes trade by selling secondhand dresses to actresses and chorus girls, which gave her an entree into the demi-monde of nightclubs and rich, louche men. Doris found herself a Viscount - Valentine Browne, Viscount Castlerosse. They married in 1928. He was smitten, she probably wasn't.
But then came the war, which made Churchill a hero and Doris's new home of Italy enemy territory. She fled with Margot Hoffman to New York. Margot, though, tired of her and her behaviour. Alone, far from home, and with her looks now fading, Doris was reduced to pawning her diamonds.
According to the Channel 4 documentary, when Churchill went to America to see President Franklin Roosevelt in the summer of 1942, the two "former lovers" dined together in Washington. But, Toye and Dockter claim, Churchill came to worry about that portrait - the one of the younger Doris reclining on the chaise longue - possibly falling into the wrong hands, leaving him vulnerable to blackmail or the subject of a damaging newspaper story.
It is believed that in September 1942, one of Roosevelt's advisers, Harry Hopkins, arranged for Doris to return home on a ship to England. Viscount Castlerosse was waiting for her in London. According to Lyndsy Spence he had written to Doris offering to remarry her.
When she stepped off the train, in the dark of the wartime blackout, all was tender reconciliation between them. But, says Spence, when the viscount saw the older, faded Doris by the light of a Dorchester hotel room he was so shocked by her haggard appearance that he scurried back to Enid, Viscountess Furness, "an Australian wine heiress and serial widow".
Running out of options, Doris sent a telegram to the New York pawnbroker, telling him to sell her diamonds. Little did she know that selling diamonds during wartime was illegal and her telegrams were being intercepted by Scotland Yard. Doris was visited by detectives, and left fearing a jail sentence.
And now, in the eyes of the high society that had once embraced her as the most ecstatically beautiful woman in London, Doris had resumed her natural status as a "common little demi-mondaine" - as one of them called her. She was even lower than that to some of them.
By heading for America Doris had, in their eyes, deserted England in time of war - during its "Darkest Hour", to borrow the title of the latest film to celebrate her old L'Horizon companion Churchill.
In her book The Riviera Set, the author Mary Lovell recounted how, in the corridor of the Dorchester, Doris happened upon an earl with whom she had once had "a dalliance". "He cut her dead but muttered aloud as they passed that she was a traitorous b***h".
And then, one morning soon afterwards, staff at the Dorchester unlocked Doris's door with a master key and found her in bed, unconscious, an empty pill bottle at her side.
The fatal overdose left her dead at the age of 42.
Churchill's painting of her in her chaise longue glory, the documentary-makers suggest, was discreetly retrieved by Churchill's friend Lord Beaverbrook. And so the mystery of what happened between Churchill and Lady Castlerosse remains unresolved.
But having heard of her life and her cruel end who, beside perhaps a Churchill relative and a historian or two, could begrudge this remarkable woman her moment of posthumous fame?
Churchill's Secret Affair, Channel 4, Sunday, 8pm