Prince William and Prince Harry’s relationship was damaged years before Meghan Markle entered the picture, a royal author says.

“People didn’t really acknowledge the fact that there was all this tension between Harry and William ever since they were small children, wrestling in the back of the car,” Christopher Andersen, author of “KATE! The Courage, Grace, and Power of the Woman Who Will Be Queen,” told Page Six in a recent exclusive interview.

Andersen added that it was “made very clear” to Harry that “he was lesser than,” and that “left a real mark on them and really kind of doomed their relationship, ultimately.”

Harry, 41, detailed the different ways the siblings were treated in his explosive memoir, “Spare,” claiming that William, 43, was given a much nicer bedroom in Kensington Palace.

“My half of the room was far smaller, less luxurious,” Harry wrote in the book. “I never asked why. I didn’t care. But I also didn’t need to ask.”

Harry also claims his brother “appeared supremely irritated” when he first heard of the idea for the Invictus Games, which Harry created. He blamed William’s less-than-excited response on their “rivalry.”

Their relationship deteriorated once Harry began dating Meghan Markle.

Harry writes that the two of them had an argument over unflattering press reports about Markle’s behavior, which quickly turned physical, with William pushing Harry to the ground.

The brothers have reportedly not spoken to each other in several years.

Andersen tells Page Six that William’s wife, Kate Middleton, 44, often tried to “get them back together.”

One example of her peacemaker inclinations was after Prince Philip’s funeral in 2021, when she was walking with William and diplomatically broke away when Harry joined them, allowing the siblings to chat privately.

“She wanted them to make amends,” Andersen says.

Spokespeople for both princes did not return Page Six’s emails for comment.