The Duke of Sussex claims things got physical between him and Prince William back in 2019 ... with William launching the attack.

In his new memoir, "Spare," Prince Harry claims William grabbed him by the collar, ripped his necklace and knocked him to the ground during a fight in London.

As for what sparked the alleged fight ... Harry says things got heated after William called his wife Meghan "difficult," "rude" and "abrasive."

Prince Harry says William was already worked up when he came over to Nottingham Cottage, where Harry was living at the time, with William starting to complain about Meghan.

Harry says he told his brother he was just regurgitating media narratives and he expected better from William ... and then they got into a shouting match before things ultimately got physical.

Prince Harry says he accused William of acting like an heir and being unable to understand him ... and they exchanged insults, then went into the kitchen, where the alleged attack happened.

Harry goes on to say the alleged fight left him with visible injuries because he fell and landed on a dog bowl, which cracked under his back ... according to excerpts of the book obtained by The Guardian.

What's more, Harry claims William urged him to hit him back, bringing up fights they had when they were younger ... but Harry says he refused to strike his brother. William then bounced, but came back "looking regretful" and apologized.

Prince Harry also says Meghan noticed the bruises and scrapes he suffered in the alleged fight and that's when he told her what happened. He says MM wasn't surprised nor angry.

Prince Harry's memoir is due out next week ... and you can imagine the royal family won't be too happy about some of the contents.

The timing is interesting ... Harry is hoping to mend a deep rift within the royal family, recently telling the UK's ITV he wants his father and brother back in his life.