Prince Harry has claimed his brother William physically attacked him, according to the Guardian, which says it has seen a copy of the Duke of Sussex's memoir, Spare.
The newspaper reported that the book sets out an argument between the pair over Prince Harry's wife Meghan.
"He grabbed me by the collar, ripping my necklace, and he knocked me to the floor," the Guardian quotes Harry.
Kensington Palace and Buckingham Palace have both said they will not comment.
The palaces - which represent Prince William and the King respectively - seem to have adopted the strategy that any controversial claims will fizzle out faster without a response.
Meanwhile, in a new clip previewing an interview with ITV, Prince Harry refuses to commit to attending the King's coronation in May.
He says there is a lot "that can happen between now and then" and the "ball is in [the Royal Family's] court".
Prince Harry's memoir will not be published until next Tuesday, but the Guardian said had it obtained a copy amid what it called "stringent pre-launch security".
BBC News has not yet seen a copy of Spare.
The book has, however, accidentally been put on sale in Spain five days ahead of its anticipated publish date - entitled En La Sombra, which translates as "In the shadow".
Book shops in the UK say they are under a strict embargo to ensure the autobiography is not released early.
According to the Guardian, the book claims the row was sparked by comments Prince William made to Prince Harry at his London home in 2019.
Prince Harry, the paper says, writes that his brother was critical of his marriage to Meghan Markle - and that Prince William described her as "difficult", "rude" and "abrasive".
The Duke of Sussex reportedly writes that his brother was "parrot[ing] the press narrative" as the confrontation escalated.
Prince Harry is said to describe what happened next, including an alleged physical altercation.
"He set down [a glass of] water, called me another name, then came at me. It all happened so fast. So very fast.
"He grabbed me by the collar, ripping my necklace, and he knocked me to the floor.
"I landed on the dog's bowl, which cracked under my back, the pieces cutting into me. I lay there for a moment, dazed, then got to my feet and told him to get out."
Harry writes that William left but returned "looking regretful, and apologised", the Guardian says.
When William left again, Harry is said to write that his brother "turned and called back: 'You don't need to tell Meg about this.'
"'You mean that you attacked me?'
"'I didn't attack you, Harold,'" Prince William is said to have responded.
Harry's name is not short for Harold - his actual full name is Henry Charles Albert David.
Photographs suggest Harry regularly wore a dark necklace at events such as the Invictus Games, and on foreign tours with Meghan, as recently as September 2019.
The revelations create the bleak impression of a family fight, right at the centre of the monarchy, that shows no sign of being reconciled.
This is still the territory of an acrimonious divorce rather than the reconciliation.
Separately, the memoir claims Prince William "howled with laughter" when he saw his brother dressed in a Nazi costume before a fancy dress party in 2005, the New York Post reports.
Harry was 20 when a picture of him in the outfit was published in the UK press.
The New York Post reports Harry asked William, and his future wife Catherine, whether he should wear the costume, or dress as a pilot - and claims the pair laughed and said the Nazi uniform.
Martin Pengelly, a journalist for the Guardian's US website who wrote its report on Harry's book, said he had not approached Prince William's communication team.
The reporter said that his article was "a report on Harry's book, which he's written - it's Harry's account".
Mr Pengelly told BBC Radio 5 Live: "We carefully, obviously in reporting it, didn't call it a fight because Harry says he didn't fight back."
While publishers at Penguin Random House are yet to confirm whether the leaked excerpts from the book are genuine, Prince Harry has recently spoken of his troubled relationship with his brother.
And the duke calls William his "beloved brother and arch-nemesis" in his memoir, an interview with Good Morning America reveals.
In that interview, Harry says there has "always been this competition" between the pair, and it played into the "heir/spare" dynamic which formed the basis of the book's title.
The concept of the "heir and the spare" dates back centuries in royal circles and refers to the continuation of the royal bloodline: the first son and heir the one who inherits the throne, the second son therefore a spare should anything happen to the first-born.
In Harry and Meghan's Netflix documentary, the prince recounts a meeting he attended with his brother, father - the now King - and the late Queen, his grandmother.
Describing the conference in early 2020, he says: "It was terrifying to have my brother scream and shout at me and my father say things that just simply weren't true, and my grandmother quietly sit there and sort of take it all in."
The Guardian says Prince Harry details a meeting with Charles, then Prince of Wales, and Prince William after the funeral of his grandfather, Prince Phillip, in April 2021.
If this leak is accurate, perhaps the most poignant image is of King Charles caught in the middle, asking his warring sons not to make his life a "misery".
Spare, ghostwritten by memoirist JR Moehringer and part of a multi-million dollar book deal, was previously believed to be subject to the utmost secrecy with few details known about its content.
"For Harry, this is his story at last," Penguin Random House said in a publicity statement back in October.
Archewell, the Duke and Duchess of Sussex's company, has not commented on news of the book.
In a trailer for a sit-down interview, which will be broadcast on 8 January ahead of the book's release, Prince Harry said: "I would like to get my father back, I would like to have my brother back".
However, Harry told ITV's Tom Bradby "they've shown absolutely no willingness to reconcile," although it was not clear who he was referring to.
Buckingham Palace has declined to comment on this.
BBC
Russia's unmanned Luna-25 spacecraft has crashed into the Moon after spinning out of control, officials say.
Donald Trump has confirmed that he will not take part in Republican presidential debates with his rivals in the race for the White House.