Prince William 'furious' over new attack on Princess Kate

Scobie previously wrote Finding Freedom, a biography of Prince Harry and Meghan Markle, but the Sussexes have distanced themselves from his latest work.

Scobie told the Sunday Times regarding the Princess of Wales: "In the coverage of Kate we infantilize her massively so the bar is always lower.

"The small achievements that we’ve seen from the Princess of Wales wouldn’t perhaps be noticed if it was from another member of the royal family, but with Kate it’s like ‘wow!'"

An insider close to Prince William has said the criticism of Kate will anger the royal.

A source told the Mail: "That will really wind William up."

They added: "He's immensely proud of what his wife has achieved.

"The princess has been the driving force behind a great deal of their work, particularly when it comes to bettering mental health."

An ally of Prince William has torn apart fresh claims on Meghan Markle and Prince Harry's treatment in the Royal Family.

A source close to William told the Mail: "It's one thing writing a critique of the Royal Family. That's freedom of expression.

"But it's another thing to peddle conspiracy theories dressed up as fact.

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"The suggestion that [royal aides] were briefing negative stories [about Harry and Meghan] is totally fabricated. It just didn't happen.

"The truth is that everyone was walking on eggshells practically from the engagement trying to keep them happy."

Earlier this month a royal insider said that Kate, Princess of Wales and William appear to have been targeted by Scobie in Endgame.

They told the Mirror: "Nothing is off limits with William and Kate appearing to be this author’s number one target."