Harry & Meghan on Netflix
The six-part docuseries "Harry & Meghan" premiered on Dec. 8, 2022, just three months after King Charles III ascended the throne. Harry & Meghan/Netflix

King Charles III was left fuming after Prince Harry and Meghan Markle released their Netflix documentary "Harry & Meghan" just a few months after his ascension to the throne.

In an advance copy of his book "Endgame: Inside the Royal Family and the Monarchy's Fight for Survival" obtained by Page Six, royal author Omid Scobie quoted a source who said the show "took the wind out of everyone's sails".

The insider added that the release of the docuseries left His Majesty fuming because he was "doing his damnedest best" to get positive media attention as the new monarch. However, "no one (was) watching thanks to the Sussexes' highly publicised Netflix series 'Harry & Meghan'".

Another royal insider told Scobie that in his frustration, King Charles III "went from not wanting anyone to talk about his son to openly criticising" him. He allegedly referred to Prince Harry as "that fool".

"The King was genuinely sad about the entire situation. He was angry but didn't want people to speak ill of his son in front of him, either," the source told the author three months after the series premiered on Netflix on December 8 last year.

The insider added: "It was a brief moment where he (King Charles III) paused and realised how bad things had become."

Elsewhere in his book, Scobie alleged that "both the institution and the family" struggled to "come to grips with (the) fact that Prince Harry and Meghan Markle were successful in their own endeavours in America".

He wrote: "I've watched Kensington Palace and Buckingham Palace repeatedly get worried about being upstaged and derailed by the couple, and leak negative information on Harry and Meghan ... during moments that could easily be focused on genuine royal news instead."

The journalist claimed that His Majesty's sympathy for the Duke and Duchess of Sussex "soon disappeared" when their media attention overshadowed his own. It also did not help fix his strained relationship with the royals when the duke released his memoir "Spare" just a month after "Harry & Meghan" premiered.

Prince Harry released his book earlier this year on Jan. 10 and he also did a series of promotional interviews, including with CNN's Anderson Cooper and with ITV's Tom Bradby. In these interviews, he shared that he never intended to hurt his family with the contents of his memoir because he only wanted to share his truth. He said he will always love and forgive them. According to Scobie, "Spare" was the duke's "last attempt at telling his family how he's felt for years because clearly there's never been an open enough forum to have these conversations or share these feelings".