Camilla was certainly not on any list of eligible spouses as Charles searched excruciatingly publicly for a bride. Presumably tired of waiting, she went off and married Andrew Parker-Bowles.

Many kings have had mistresses — not a few bequeathed illegitimate children, too — but very few have married and so made them queen.

King Charles III’s wife, Camilla, is therefore treading new and potentially sensitive ground as his Queen, as she is well aware. But social attitudes have changed considerably in the twenty-five years or so since she emerged from behind the shadow of Diana, Princess of Wales, as what the latter described as “the third person” in her marriage during her 1995 Panorama interview.

Had royal attitudes evolved earlier fifty years ago, when Charles first courted her, and accepted the possibility of the heir to the throne marrying a commoner instead of a princess or a member of the aristocracy — as his son William was eventually able to do — then things might have been a lot less troubled for the royal family.

Not that Camilla Rosemary Shand, as she was born, is particularly common. Her mother, Rosalind, was the daughter of the 3rd Baron Ashcombe and her father, Bruce, was a former major who had become an upmarket wine merchant after leaving the army. One of her maternal great-grandparents was Alice Keppel, Edward VII’s mistress.

Camilla and her younger brother and sister grew up in East Sussex and central London, the children of privilege and affluence. In the words of the royal biographer Gyles Brandreth: “The Shands without question belonged to the upper class, had position … they opened their garden for the local Conservative Party Association summer fete. Enough said.”

She had a private education — no university — and went to a Swiss finishing school, rounding off with a French course in Paris. She was fun and had fun. She was fired from one job for coming in late after a party, working for the fashionable decorating firm of Sibyl, Colefax and Fowler in Mayfair as a receptionist.

A debutante in 1965, she became one of the group of affluent young women moving in similar social circles to Charles, who was eighteen months her junior. They shared common interests and he was apparently smitten. They had a covert relationship, occasionally snapped by the press during furtive trysts at polo matches.

But somehow Charles hesitated. He went off on naval service. Was she not royal enough? Was he not entirely sure?

Was it all becoming too public? “They were ideally suited, we know that now, but it wasn’t possible,” said Charles’ cousin Patricia Knatchbull.

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Camilla was certainly not on any list of eligible spouses as Charles searched excruciatingly publicly for a bride. Presumably tired of waiting, she went off and married Andrew Parker-Bowles, a Guards officer in the Blues and Royals, in a society wedding in 1973. The marriage produced two children, Tom and Laura, but ended in divorce in 1994.

Meanwhile, Charles’ supposedly fairy-tale marriage to Diana Spencer, a member of one of the oldest aristocratic families in England, was also floundering, first privately and then increasingly publicly. The couple shared few interests, Charles was thirteen years older than his wife and the couple had scarcely known each other before their glittering wedding. Charles was jealous of Diana’s popularity and she found him distant and unaffectionate. She increasingly came to blame Camilla for their growing estrangement and Charles admitted adultery.

Charles and Camilla had got back together as lovers in 1980, the year before his wedding to Diana, resumed again in the mid-80s and in 1992 their affair became public with the publication of the so-called  Camillagate tapes, secretly recorded intimate conversations between them in which Charles famously wished he could be her tampon. The prince refused to end the relationship and in the mid-90s he and Diana and the Parker-Bowleses both divorced.

After the Panorama interview and Diana’s death in a Paris accident, Camilla was widely vilified as if she had been responsible for the breakup of the prince’s marriage and the princess’ death.

A slow and coordinated public relations exercise by Camilla and the prince followed, orchestrated by Charles’ advisers to make the relationship appear appropriate and improve her public image. They were seen together, meeting at events and gradually Camilla began to accompany Charles. She was introduced to the Queen in 2000 and seen publicly in the company of the monarch during her golden jubilee celebrations in 2002.

A cautious assessment of public attitudes was commissioned. Would the public accept her after what had happened to Diana? Public attitudes were changing. Divorce and even adultery were no longer necessarily public taboos and in 2005, when preparations were made for the couple to marry, the only objections came from the conservative evangelical fringes. The Church of England was not prepared to marry the couple outright, but the then archbishop of Canterbury, Rowan Williams, agreed to conduct a blessing after a registry office wedding in Windsor.

The marriage appears to have been a success. It has made Charles visibly less tense and grumpy and more smiley at public events, and Camilla has come to be regarded as a good sort friendly and approachable. Now at last she is what Charles always hoped and planned for, his Queen.