More than half a century after they met, and following 21 years of marriage, Camilla remains the King’s greatest supporter. She is his closest confidante, most trusted aide and the person who always knows how to make him laugh.

Her support has never wavered, even in their darkest hours. Goodness knows, it has been far from easy.

But now she faces a challenge that will eclipse all others. As the author of Camilla: From Outcast To Queen Consort, I fear that the return of the notoriously difficult Sussexes to Britain could be the test of her lifetime.

Of course, those early days of marriage were tough, too. On the morning of her royal wedding in Windsor in 2005, Camilla was so afraid of the public reaction to her nuptials that she initially refused to get up at all.

The public still adored the late Diana – and I will never forget a close friend of our now Queen telling me that Camilla feared having rotten fruit hurled at her that day, with crowds jeering.

In fact, it took her sister Annabel Elliot to convince her to get out of bed and put her best foot forward.

I believe she made a decision that morning to approach every challenge with resolve and dignity.

She is now an integral part of the royal operation and, of course, has even won over the public. A devoted and loving mother to her two children, Tom and Laura, and the bedrock of her beloved husband’s reign.

But there has always remained the tricky issue of her relationship with the King’s two sons, and Harry in particular.

For years, she suffered the public ridicule of being the ‘other woman’ to the boys’ much-missed mother.

Harry, meanwhile, could never conceal his disdain for the woman his father had loved long before he married Diana.

When Harry married Meghan in 2018, Camilla tried to help the American actress negotiate royal life, having once been an outsider herself. She offered to take Meghan for private lunches and tried to give her advice on how to handle the pressure of being in The Firm.

But a close friend of Camilla’s told me that Harry’s new bride seemed unenthusiastic about the gestures, and the advice ultimately seemed to land on stony ground as Meghan chose to navigate the role on her and Harry’s own terms.

As for Harry, we need look no further than the media campaign he waged against the Royal Family as he promoted his appalling 2023 memoir, Spare. In an interview with the US show 60 Minutes, he deemed her ‘dangerous’ because of her connections with members of the media. He called her a ‘villain’ and even declared: ‘I don’t look at her as an evil stepmother. I see someone who married into this institution and has done everything that she can to improve her own reputation and her own image for her own sake.’

This is despite the fact Camilla’s record is one of dutiful service, as shown by her numerous charity endeavours. I’m told that when the Sussexes visited Britain in July and met the King at Highgrove, Camilla insisted on being present, much to Harry and Meghan’s ire. But she was right to do so. She knows full well that Harry can be a tricksy individual and headstrong, in comparison to the King’s more gentle manner.

All the Queen wanted to do was support the King in what would have been a difficult meeting.

Harry’s coldness towards Camilla feels all the more unkind considering the now 79-year-old Queen and her 77-year-old husband had hoped to put old rivalries behind them and spend their twilight years in dutiful service and quiet reflection.

Worse still, of course, the King is still being treated for cancer. The past few years have been demanding for the royal couple, juggling his treatment with public appearances, charity work and constitutional affairs.

In an interview released just this week, Camilla admitted that it had been ‘quite difficult’ keeping her husband’s diagnosis private. Not least because she opened a cancer support centre for Maggie’s cancer charity at the Royal Free Hospital in London in January 2024, a week before the public knew of the King’s condition.

‘I was just longing to let it out,’ she revealed, ‘but obviously I couldn’t.’

Despite his doctors advising the King to slow down and work less, he insists on keeping up with royal duties. And this is where Camilla is invaluable. She is the one who suggests when it would be a good idea to rest and when to let him crack on. They have found a way to make it work.

But now I fear the Sussexes’ return will add another level of stress to the King’s life.

During the Covid years, Camilla and the late Queen Elizabeth became very close – despite their admittedly difficult start – calling each other on Zoom every morning for a small slice of human connection.

Indeed, Camilla was pictured at York races this week wearing the late Queen’s brooch, in a moving nod to her mother-in-law.

I have no doubt that Queen Elizabeth would be deeply upset at her grandson’s behaviour, but she would have kept her counsel. As will Camilla. She is sensible and intelligent.

And while King Charles continues with his duties, so, too, will Queen Camilla, carrying on as she did on the morning of her wedding: always putting her best foot forward.