Prince Harry received a brutal warning over his memoir, Spare, as historian dubbed the book "horrible" and said that the Duke has lost his credibility.

In a conversation with GB News, historian Simon Heffer criticized the Duke of Sussex’s attacks on Royal Family members, especially King Charles and Prince William, in the book.

He noted that Harry made a mistake by airing his dirty laundry in public and cautioned Harry that he must "play by the rules" and stop embarrassing the Royal Family if he wants to be welcomed back.

Heffer was asked if he had read Harry’s bombshell book, to which he said, "I read bits of it. I mean, life's too short, but I read enough of it to get a flavour of it. I thought it was horrible."

He was then asked, "Do you think that we should accept him back to the UK if Hollywood don't want him?"

"It's not up to us,” the expert responded. “The decision has to be made by his father and with the advice of his brother.”

"I think if things go wrong for him there, and he's willing to come back and play by the rules that the rest of the Royal Family play then it would be wrong of any of us as mere subjects, to say we shouldn't come back.

"But there's no doubt that if he continues to embarrass the Royal Family and to make a bit of an exhibition of himself, the public won't like it, and he will be a drag on the Royal Family."