New book Finding Freedom claims Prince Harry was left "p***ed off" after Prince William warned him not to be "blindsided by lust" and urged him to "take as much time as you need to get to know this girl".
This allegedly rattled Harry who accused his brother of taking a "tone of snobbishness".
But royal expert, Richard Fitzwilliams, claims the alleged remarks are unfair on William who he says sounds like he was simply watching out for him.
Mr Fitzwilliams told Daily Star Online: "I think it's ridiculous, I think it's brutally unfair, I don't think it's necessary.
"Firstly we have no way of verifying the quote, secondly if that is what he said then what he was trying to do was just make sure Harry made the right choice as one might do with a younger brother.
"The facts simply are that they had a whirlwind courtship, it was certainly far quicker than Kate and William, no question about that.
"But we learned all these details about the early days, and the only way we can learn them was through members of their inner circle."
Harry and Meghan deny any involvement in the book, which claims to have spoken to people close to the couple.
A spokesperson for the Duke and Duchess of Sussex said: "The Duke and Duchess of Sussex were not interviewed and did not contribute to Finding Freedom.
"This book is based on the authors' own experiences as members of the royal press corps and their own independent reporting."
But Mr Fitzwilliams suspects they could have blocked the book if they wanted to.
He added that it may have also had a negative affect on the Queen and dad Prince Charles.
Mr Fitzwilliams continued: "I think they will think it's very unfortunate, I think they will think it's very sad."
He added: "The timing is bizarre if they wish for public sympathy which is presumably the purpose of it, setting the record straight.
"Who are the sources exactly? All I can say is I am distinctly underwhelmed by the extracts."
Harry and Meghan sensationally stepped down as senior royals at the start of this year.
They left the UK to live in Los Angeles with baby Archie via Canada, but have not returned to the UK since due to the coronavirus pandemic.