As Camilla grew older she’d lie under the umbrella-shaped gazebo in the garden, after which the house is named, and devour books taken from the well-stocked library.

Among them were Broderie Anglaise, written by Camilla’s great-aunt Violet Trefusis, and Challenge by Vita Sackville-West.

Through their pages, the future queen came to learn about one of the greatest unspoken society scandals of the 20th century – the scorching lesbian affair between the two authors who eloped together, leaving their husbands behind.

Violet was the second daughter of the fabled Alice Keppel – the last and most powerful of King Edward VII’s mistresses. Violet liked to say that she was the King’s daughter, though she wasn’t (her sister Sonia, Camilla’s grandmother, may have been).

It was Edward VII’s money which had bought Ombrellino and where, for decades after the King’s death, Alice reigned as queen.

But her daughter Violet was a wayward and unruly child who, when sent away to boarding school, got to know Vita, daughter of the wealthy Lord Sackville.

At 14, Violet declared her undying love for the older girl – and the seeds of the scandal which would last many years were sown.

 Alice Keppel – the last and most powerful of King Edward VII’s mistresses and Queen Camilla’s great-grandmother

Queen Camilla, her brother Mark and sister Annabel with their mother Rosalind. They frequently went on holidays to beautiful Tuscany while they were growing up

Villa dell’Ombrellino, a palatial mansion in Tuscany which was bought with Edward VII’s money

Camilla in Florence with the then Prince Charles

Brunette, beguiling, and always with a rebellious look about her, Violet captivated Vita. And though, as they emerged into adulthood, Vita became engaged to diplomat and writer Harold Nicolson, the women maintained an unbreakable bond.

Jealous of Vita’s forthcoming marriage, Violet became engaged to Denys Trefusis, a mild-mannered aristocratic soldier. But before the wedding could take place, she and Vita disappeared to France together for several months, and Violet told her future husband she could only go through with the marriage if he promised they would never have sex.

Extraordinarily, Denys agreed.

Within a day of their arrival in Paris after the Trefusis’s Mayfair wedding in 1919, Violet and Vita met at the Ritz Hotel and, according to Violet’s celebrated biographer Diana Souhami, had sex. ‘I treated her savagely. I had her. I didn’t care, I only wanted to hurt Denys,’ Vita wrote afterwards.

The next day they summoned Denys to tell him his marriage was a sham. ‘Violet told him she had meant to run away with me instead of marrying him; she told him she didn’t care for him. He got very white, and I thought he was going to faint. I wanted to say, “Don’t you know, you stupid fool, she is mine in every sense of the word?”,’ Vita wrote in her diary. The soldier, who’d just returned from the Great War having been awarded a Military Cross, broke down in tears.

‘He had been used, tricked,’ writes Souhami.

Alice Keppel, mistress of King Edward VII, pictured with her daughter Violet in 1912

Jealous of Vita’s forthcoming marriage, Violet (above) became engaged to Denys Trefusis, a mild-mannered aristocratic soldier

Violet married Denys Trefusis in Hanover Square, London, but within a day of arriving in Paris after the wedding, Violet and Vita met at the Ritz Hotel and, according to Violet’s celebrated biographer Diana Souhami, had sex

The novelist and poet Vita Sackville-West was married to the diplomat Harold Nicolson but was also Violet’s lover

Broderie Anglaise was written by Camilla’s great-aunt Violet Trefusis

Husband and wife quit Paris and travelled south, but when they arrived at Saint-Jean-de-Luz they slept in separate hotel rooms.

Meanwhile, Vita was sending letters from Paris, determined to ensure Violet and her husband were not having sex.

Back home in England, the lovers could not bear to be apart and audaciously planned another elopement that autumn. Meanwhile, to console himself, Vita’s husband Harold Nicolson found himself a new lover – the couturier Edward Molyneux (later to dress the Duchess of Windsor).

The lesbian relationship was ‘not just a torrid affair’, writes Souhami. They had loved each other for more than a decade.

But when Violet suggested buying a house in Sicily with Vita and moving there permanently, Vita got cold feet – she sensed social isolation and, maybe, bankruptcy. She engineered a situation where her husband and Denys would hire a plane in London and fly to France to ‘rescue’ her.

There was an angry scene between the runaways and their husbands at a hotel in Amiens, but nothing changed and everybody carried on as before. The women were too sexually and emotionally entangled to part.

But now there grew a difference between the two lovers. Violet was prepared to throw away everything – her reputation, money, husband, future – for Vita. But Vita wanted to cling on to what she’d got.

The lovers separated, came together again, then separated once more. It was a recipe which repeated itself several times more in the coming years, but the wheels were slowly coming off the relationship.

Because of the affair Violet and Denys had parted, but Vita and Harold did not.

Mrs Keppel, who held the purse-strings, ordered Violet to find somewhere to live in Italy, and in the end she came to live with her mother at Villa dell’Ombrellino. ‘There was never enough furniture and it was a real white elephant, with two sitting rooms to every bedroom,’ recalled a visitor. ‘But as far as Alice Keppel was concerned, it was Buckingham Palace.’

King Edward VII was Alice Keppel’s lover – she was his last, and most powerful, mistress

Alice Keppel spent many years after the death of King Edward VII in a palatial Florence mansion, the Villa dell’Ombrellino, which overlooks the city

Violet liked to say that she was the King’s daughter, though she wasn’t

But when Alice died – just two months after her grand-daughter Camilla was born – Violet inherited the house. And here, throughout her childhood, Camilla and her siblings would come to play every summer, delighting in the lush surroundings and spacious grounds.

When Camilla returned to Ombrellino a few years ago – now empty and mothballed – she mournfully expressed the hope that something could be done to bring the house back to life. But today it still stands empty.

Like her great-aunt, Camilla was to create a scandal all of her own – though one which ultimately resolved itself happily.

The same, alas, could never be said of Violet, who died aged 77 in 1972, the tragic victim of what she described as ‘the unwanted licence of loneliness’.

Vita had two children and lived a rich life as an author and garden designer. But never again in her life was Violet able to find someone to love and cherish.

Read more in Diana Souhami’s entrancing account of the life of Violet Trefusis, Mrs Keppel And Her Daughter (Flamingo)