Piers Morgan has said he will sue Earl Spencer over “fabricated inaccuracies” and “examples of brazen falsity” in his book about his late sister Diana, Princess of Wales, as the publication hits shelves.

The earl has made a series of claims in Swan Song: Diana, My Sister – including that the King said “we’ll forget her soon enough” after she died and he sounded “giddily elated” in the wake of the crash – which has prompted a war of words between him and Buckingham Palace.

The book, which is out now, also contains allegations about Morgan, which the broadcaster has strongly rebuked.

Speaking on his YouTube show Piers Morgan Uncensored, he said: “Many allies of His Majesty have called into question these facts, ever since the most salacious accusations were serialised by the Daily Mail.

“Many royal commentators and experts have lined up to say that they simply don’t ring true. The King himself issued a furious rebuke that is almost without precedent. But, in essence, this has all been about opinions. Recollections may vary, after all. Well, until now.

“Because I can exclusively reveal on Uncensored that Earl Spencer’s book contains fabricated inaccuracies which will strike to the heart of his credibility. Spencer recounts how he personally called round the tabloid newspaper editors to uninvite them from Princess Diana’s funeral, after they had all been invited.

“I remember this well, because I was one of them.”

Morgan read an extract from the book, which said: “They realised they were unable to be rude to the brother of the much-mourned princess… The only one who sounded surly, as opposed to merely confused or embarrassed, was Piers Morgan of the Mirror, who snarled with annoyance before he hung up.”

“I assumed Morgan must be feeling particular guilt because, four years earlier, he and his Sunday Mirror counterpart had published photographs taken with hidden cameras by a gym owner. They were of Diana working out in a leotard.

“This was such an intolerable invasion of her privacy that Diana instructed lawyers to sue the Mirror Group and to block further use of the images.

“Meanwhile, he says, a Dutch newspaper splashed one of the pictures on its front page, and an American tabloid bought a selection of them with which to titillate its readers. The gym owner eventually made £300,000 from his salacious, stolen images.

“Spencer continues, for a while it looked like the Mirror group would be brazen enough to allow the case to go to court.

“Spencer continues, it agreed to pay Diana’s legal costs of around £1 million, and was obliged to further acknowledge its guilt by making donations of £200,000 to her chosen charities.

“He then says, in the immediate aftermath of Diana’s death, Morgan was one of four tabloid editors named, shamed and photographed in an article looking at who might be held responsible for the paparazzi’s role… It must have been an extraordinary sensation for this powerful quartet…They were used to dishing out such treatment, without ever receiving it in return.”

Addressing his audience, Morgan remarked: “Well, this is all pretty extraordinary stuff, isn’t it? And very damning, particularly about me.

“I come across in the most despicable manner imaginable. There’s just one problem. It’s all bullshit and very, very easy bullshit for me to prove it.

“The photographs which he says that I published were printed by the Sunday Mirror on November 7, 1993. I joined the Daily Mirror in October in 1995.

“In fact, at the time of the gym pictures, I was doing a showbiz column on the Mirror’s arch-rival, The Sun. About as far removed from responsibility for the Mirror scoop as I could possibly have been.”

He continued: “Earl Spencer was estranged from Diana and living in South Africa at the time of her death.

“As editor of the Daily Mirror, I had lunch with her at Kensington Palace with Prince William. After that, it wasn’t uncommon for her to call my office or for me to run stories by her… Ironically, by that time in her life, I think I was probably talking to her more often than he was.

“My question though is this, if Earl Spencer can be so incredibly, defamatorially wrong about such basic facts involving me, well, why should we believe anything else he puts in this book?”

He accused the earl of “using the Harry and Meghan playbook” of “putting their truth before the truth”, continuing: “I expect there will be many more examples of brazen falsity after what I’ve uncovered… In the meantime, Earl Spencer, you’ll  be hearing, and your publishers, from my lawyers.

“Because I would like the complete lie that you’ve published about me, and those photographs, to be removed from your book.

“And I would like a donation to be made to the King’s Trust for the damage that you’ve caused with your lie, after all.

“To quote somebody, you can’t dish it out without getting it in return, right?”

Earl Spencer has been contacted for comment.