PRINCE William physically attacked his brother Harry, knocking him to the floor, after he attempted to stand up for his wife Meghan Markle, the younger prince has alleged.

Harry makes the accusation in his upcoming book Spare, which is expected to contain more than a few headaches for the royal family, according to The Guardian.

The memoir, which is due to be published on January 10, has been seen by that paper, which has reported on some of its key revelations.

It details one confrontation between Harry and William in 2019 at Nottingham Cottage in the grounds of Kensington Palace – where the younger brother was then living.

READ MORE: Tory MP plots law to strip Harry and Meghan of royal titles following Netflix show

Harry claims that William called Meghan Markle “difficult”, “rude” and “abrasive” – which led him to accuse his brother of parroting the British tabloids' narrative about her.

Harry said William was “piping hot” when he had arrived at the cottage, and after a verbal exchange he had told his older brother: “Willy, I can’t speak to you when you’re like this.”

The book goes on: “He set down the 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 goes on to claim that William told him Meghan did not need to know about the incident, and denied that he had “attacked” his brother.

Harry said Meghan was “terribly sad” to learn of the incident, which he did not tell her about immediately but only when she saw the scrapes and bruises on his back.

The title of the book comes from the idea that royals would need an heir – the oldest son – and a “spare” to take the throne should anything happen to them.

Early in the book, the Guardian reports, Harry tells of how his own father labelled him the spare on the day of his birth.

“Wonderful! Now you’ve given me an heir and a spare – my work is done,” Charles reportedly said to Diana back in 1984.

The National: Kate and William

The book also alleges that it was William and his wife Kate who encouraged him to wear the now-infamous Nazi uniform to a fancy dress party in 2005 – saying they “howled” with laughter when they saw it, according to US website Page Six.

Harry claims he phoned the couple to ask them whether he should choose a pilot’s uniform or a Nazi one for the fancy dress party and William and Kate said the latter.

Harry, then 20, sparked outrage and was widely condemned when a photograph of him in the uniform complete with a swastika ended up on the front page of a newspaper.

He said in his Netflix documentary: “It was probably one of the biggest mistakes of my life.”

Other revelations from the book include how the brothers call each other “Willy” and “Harold” and that Charles pleaded with his sons during a tense meeting after the Duke of Edinburgh’s funeral: “Please, boys. Don’t make my final years a misery.”

Kensington Palace and Buckingham Palace have declined to comment on the leaked claims from Harry’s book.