Piers Morgan has threatened legal action against Earl Spencer over what he claims are "lies" in his new book.

In Swan Song: Diana, My Sister, Spencer writes about being "flabbergasted" to learn that newspaper editors had been invited to her funeral.

The earl says he told the Queen's senior private secretary, Robert Fellowes, that they were not welcome as that had, in his opinion, "killed her".

Spencer writes that he personally called tabloid editors to uninvite them from the ceremony.

"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," Spencer writes.

The earl also claims that he assumed Morgan must be feeling "guilt" over the Sunday Mirror publishing photographs of Diana wearing a leotard in a gym four years earlier.

Morgan reacted to the claims on his Piers Morgan Uncensored show on Monday night.

"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,” he said.

Morgan said the Sunday Mirror published the pictures of Diana in November 1993 - two years before he started working at the Daily Mirror.

He added: "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?"

Morgan wrote on X: "I am taking legal action against Earl Spencer and his publishers over demonstrable lies he has written about me in his new book, which call into question his credibility as a supposed ‘historian'."

The earl's book, containing a string of explosive claims about King Charles, is released on Tuesday. Spencer said he felt it was his “duty… as an eyewitness to history” to write a new memoir chronicling the life of his sister.

The earl accused the King of gaslighting him after Buckingham Palace pushed back against claims that Charles said “we’ll forget her soon enough” after Diana died in 1997.

In response to the claims made in the memoir, a Buckingham Palace spokesman said on Wednesday: “While we do not comment on books as a matter of principle, His Majesty is mindful that the pain of fraternal grief can cloud reason, affect judgment and colour memory in ways others do not recognise, even many years after such a loss.”

Speaking about an alleged row over Diana’s funeral arrangements involving her sons, the earl said: “(The King) went ballistic, would be the right term, and started really going for my family as a whole, about their sense of duty, not being adequate enough, and ranted for a while, I have no idea how long, but it was a significant period, and I just listened.

“And then he stopped, and I thought, ‘Oh now he’s going to pull himself together and say something more appropriate’.

“And then he said, ‘We’ll forget her soon enough’.”

Charles is said to have made the comments about his ex-wife in a row with the earl over whether the then Prince William and Prince Harry should follow Diana’s funeral cortege.

In another extract from the interview with the BBC, the earl said the relationship between the press and the Duke and Duchess of Sussex “echoes” the way the media treated his sister.