Who are we to believe, as this monumental row, ignited by Earl Spencer’s hard-hitting book about his sister, Diana Princess of Wales, rumbles on?

In his latest salvo, which he insists is not a threat but a fact, he says he has “plenty more” he could say about the King. That certainly sounds ominous, but he contends that he’s not a salacious writer and has no interest in saying any more. The Palace will breathe a sigh of relief at that.

Charles Spencer clearly has said more – or at least something – to his nephews, William and Harry. In a BBC interview he said the book, in which he claims Prince Charles sounded “giddily elated, like a lottery winner” on the night of Diana’s death, would not be a surprise to them.

Well, it may not be a surprise, but it leaves them in an invidious position with the two sides of their family in open conflict.

The King’s friends anonymously protest that the “much married” (as if that is relevant) Earl peddling Diana’s remaining privacy in public is bazaar. They claim he can’t bear to see the popularity and success of Charles and Camilla as King and Queen, so he’s getting out his “crested carving knives” to attack his former brother-in-law.

Trading insults has never been part of Palace etiquette. But this strong, colourful language risks a tit-for-tat brawl – something the Royals have always tried to avoid. It’s all so deeply unsavoury.

And yet, Earl Spencer’s account of why he wrote his book is compelling. Now 62, he says he is feeling his mortality given that both his parents died in their 60s. His book is a brother’s tribute to a woman who was once the most famous, and perhaps most misunderstood, in the world. As a historian of some renown, the Earl felt he had a duty to set out his account of her life.

He also wanted to give some context to what he said in his spine-tingling eulogy at Diana‘s funeral. I am convinced by his argument that his final words were a direct rebuttal to Prince Charles’s alleged remark that Diana “would soon be forgotten”. From the pulpit in Westminster Abbey 29 years ago, the Earl, standing opposite the Royal Family, gave thanks for the life of “the unique, the complex, the extraordinary and irreplaceable Diana, whose beauty, both internal and external, will never be extinguished from our minds”. Boom!

In his various interviews publicising his book, the Earl is far more measured than some of the more sensational quotes from it would suggest. He says he’s not vengeful or bitter. But his words are lethal.

To reinforce his depiction of his former brother-in-law as heartless, he has recounted a conversation after the sudden death of his father, the last Earl Spencer. It meant that he inherited the title at the age of just 27. He says the Prince thought it was “lucky” to inherit so early. It was, the Earl quietly suggests, a “very peculiar” remark – thus doubling down on his portrayal of Prince Charles as “a middle-aged man displaying the frustrations of waiting to reign”. Another swipe with his carving knife.

I do wonder how much more a now-elderly King can take: the Andrew scandal, the Harry and Meghan saga – and now this, all while battling cancer.

Earl Spencer says he believes in the monarchy. Indeed, he believes in the monarchy of Charles III. His book seems a most curious way of showing it.

If, as he asserts, his motivation has nothing to do with money, perhaps he might consider giving all the profits to The Diana Award, the charity set up in her name. That would be a tribute, indeed.

In the end, though, this will pass. The book will sell well and the King’s reputation will be dented quite badly. I have seen times in the past when I really thought he wouldn’t be able to carry on and face the public again.

But he did. And he does. And he will.