She’d never even kissed her husband properly before they married. And so on her wedding night, Princess Marie Bonaparte was shocked by what occurred in the bedroom.

‘You took me that night in a short, brutal gesture, as if forcing yourself,’ she later wrote to her spouse, Prince George of Greece. ‘Then you apologised – “I hate it as much as you do. But we must do it if we are to have children”.’

So began the bizarre marriage of Prince Philip‘s aunt and uncle, in whose life he was entangled for the first ten years of his life.

Princess Marie, colossally wealthy, was Philip’s benefactor and saviour. It was her money which put a roof over his head when his parents were kicked out of Greece in 1922, and her money which paid for his private schooling.

Philip, his sisters and their parents, Prince and Princess Andrew of Greece, took refuge in a house on Marie’s estate in Saint-Cloud, a suburb of Paris. And as he grew up, across the shaved lawns came whispered words of the strange sexual antics of his aunt and uncle.

Prince George of Greece, the younger brother of Philip’s dad, was gay – and in love with his uncle.

The uncle, Prince Valdemar of Denmark, was 11 years older and, though chiefly homosexual, not averse to having fun with the opposite sex.

And that included George’s wife – Princess Marie.

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Princess Marie, Prince George, Valdemar (with his arm through George’s) and Valdemar’s wife

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Marie Bonaparte, the French author and psychoanalyst with her husband Prince George

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Prince Valdemar of Denmark; Czarevitch Nicholas II of Russia; Prince George of Greece; Prince Nicholas of Greece; and Prince Carl of Denmark in 1893

On one occasion, husband, uncle and wife had a threesome – ‘the deep sweet joy of a tête-à-tête à trois,’ Marie described it in her diary. ‘George, myself and sweet Valdemar.’

According to Marie’s biographer Celia Bertin, ‘The next day she noted down what she described as an evening scene – Valdemar and George entering her room where she was lying on a chaise longue, Valdemar kissing her and George refusing to kiss her the way his uncle did.’

She goes on to describe their activities as ‘these amorous games with Valdemar,’ while carefully avoiding the word incest. But, by any standards, such occasions must rank as a first in terms of royal sexual liberation.

Marie was the great-grand-niece of Emperor Napoleon I. Her maternal grandfather was the extremely wealthy developer of the Monte Carlo Casino, and from him she inherited a vast fortune.

George was the relatively poor son of King Constantine I of Greece and a naval officer when he met Marie – a meeting fixed up by their parents with a single purpose in mind, in which neither was consulted.

But there was a dark secret, hiding in plain sight.

George had been in love with his uncle Valdemar – his father’s youngest brother – since he was 14 and sent to Denmark to train for the navy. He went to stay with Valdemar, then an admiral in the Danish navy, at his home, Bernstorff Castle.

‘From the day we met, from that moment on, I loved him and I have never had any other friend than him,’ he later confessed to Marie.

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Prince Philip of Greece (second from left), later Duke of Edinburgh, with his schoolmates at the MacJannet American school in Saint-Cloud, Paris, in 1929

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Wedding of Prince George of Greece and Princess Marie Bonaparte in 1907

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Prince George of Greece with his wife, Princess Marie Bonaparte

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It was important for the couple to have an heir and along came Prince Peter in 1909

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Prince Philip as a young boy in traditional Greek costume in 1930

So much did he love his uncle, he invited him along on their honeymoon. And each summer for many years after, he’d leave the couple’s home in France and travel to Bernstorff so he could link up with Valdemar. Marie, the dutiful wife, tagged along.

Valdemar’s wife – also called Marie – was the first to explain to Marie Bonaparte the intimacy which united uncle and nephew, one so deep that at the end of each yearly visit to Bernstorff, George would burst into tears while Valdemar would become physically ill.

‘The women learned the patience not to intrude upon their husbands’ private moments,’ wrote biographer Celia Bertin. ‘During the first of these visits, Marie Bonaparte and Valdemar found themselves engaging in the kind of passionate intimacies she had looked forward to with her husband who, however, only seemed to enjoy them vicariously, sitting or lying beside his wife and uncle.’

Weirdly, on a later visit, Marie Bonaparte – desperate to find sexual satisfaction for herself – carried on a passionate flirtation with Prince Aage, Valdemar’s eldest son.

‘In neither case does it appear that George objected, or felt obliged to give the matter any attention,’ adds Celia Bertin.

Marie remained outwardly faithful to her husband. And to the outside world the marriage was a huge success, lasting fifty years. But she was the one who held the purse-strings, and in her search for satisfaction she embarked on a series of affairs

The men she bedded were a varied bunch, including Sigmund Freud’s friend Rudolph Loewenstein, French prime minister Aristide Briand, a prominent married French physician only ever referred to as ‘X’ – and her husband’s aide-de-camp, Captain Lembessis, of whom she confided to her diary: ‘Scenes in my room during the night. Ej. praecox (premature ejaculation)’.

‘Her love affairs were always decidedly unsuccessful,’ added Celia Bertin drily.

Sigmund Freud, the eminent psychologist, became her friend – but despite his considered advice and a series of peculiar operations to alter her genitalia, Marie was never able to find true satisfaction – though her lifelong quest caused her to write some powerful and thoroughly-researched books.

Meanwhile back in Paris, her nephew Prince Philip stayed at the Saint-Cloud house until when, just short of his 10th birthday, he moved to Britain to continue his education. But he always fondly remembered the kindness of Marie and Prince George – and when he married Princess Elizabeth, the future Queen, in 1947, they were guests of honour at Westminster Abbey.

They came again as honoured guests to Elizabeth’s Coronation in 1953 – though ever-unconventional Marie found more to interest her in whispered chats with her next-door neighbour in the Cathedral than in the world spectacle which unfolded before her.

Marie Bonaparte died in France at the age of 80 in 1962 and – despite their bedroom difficulties – she was reunited with Prince George in his tomb in the Greek royal burial ground at Tatoi Palace, near Athens.