Kate Middleton is "devastated" by the new biography about Meghan and Harry, while conversations between the brothers are now "stilted and difficult", according to experts.
Finding Freedom, to be released next week, reveals the rift in the royal family, writes the Daily Mail.
Royal writer Andrew Morton revealed how Kate really feels about the feud, telling New Idea: "There's no doubt the rift between Harry and William has deeply affected Kate. It upsets her."
Kate and William are at Amner Hall in Norfolk with their three children, while Meghan and Harry are living in a million-dollar mansion in Los Angeles with their young son Archie.
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The book claims the Sussexes and Cambridges grew so bitter with each other that by March they were barely speaking.
And there are fears that the book could spark fresh rifts within the royal family.
Authors wrote that Kate "did little to bridge the divide" with Meghan, but Morton pointed out that she "definitely sided with her husband" and acted as a "loyal wife".
He said the divide between Harry and William was "just a very sad situation" and that the pair have now "gone their separate ways".
Harry and Meghan insist they haven't given any interviews for the book, though authors say they interviewed over 100 sources including close friends of the couple.
The biography is already hugely successful and is the bestselling book on Amazon UK, weeks before its release.
The "explosive" book is at the top of the bestseller list as royal fans rush to pre-order copies.