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Prince William and Prince Harry's feud started years before Spare was published.
"I think the moment that we realized that there were tensions between the brothers was after I printed a story in Vanity Fair about William just urging Harry to take his time because they were engaged," royal expert Katie Nicholl told The Sun.
"Of course, William had taken many years before he proposed to [Princess Catherine] because he wanted her to be his wife. He wanted her to be able to know that this was what she wanted for her life, too," Nicholl said, referring to the Waleses' decade-long courtship.
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Nicholl said the beginning of the end for the siblings was when Harry was offended by William's protective approach.
"And I think, you know, what was meant to be well-intended brotherly advice landed really badly with Harry," Nicholl said. "And I think it was the start of the real unravelling of their relationship."
"I mean, Harry really, it really hurt him that he felt he didn't have his brother's backing over Meghan," the biographer continued.
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Harry and Meghan's whirlwind romance began in 2016, before getting engaged one year later. The duchess later suggested she felt rejected by the royal family and British media, leading to them stepping back from their senior-level roles.
William has avoided discussing his rift with Harry publicly, but his rumored concerns about the Sussexes seem to have come true.
"I think that William was, you know, like any brother, was probably just keeping an eye out for him by saying, do you think this is happening rather quickly?" Nicholl shared. "What do you really know about this girl? Should you not spend some more time getting to know her? Which had a bad reaction."
"He was really, really unhappy about that. He couldn't reconcile himself about that," she added. "Post then, it's just been a gradual unravelling of the relationship. I'd been told that at Christmas, when Harry and Meghan went to stay with William and Kate."
While Harry has returned to the U.K. to see King Charles III, the duke and his sibling have yet to reunite.
"There have been so many disagreements between myself and some of my family," Harry told the BBC. "I would love reconciliation with my family. There's no point continuing to fight anymore, life is precious."