According to renowned royal biographer Tom Bower, Kate Middleton has prevented Meghan Markle from attending King Charles' upcoming coronation, further straining the relationship between the two royal sisters-in-law.

Bower, author of the best-selling book "Revenge: Meghan, Harry and the War Between the Windsors," made the startling revelation during an interview with GB News on Thursday. He suggested that Markle may have wanted to be present at the historic occasion. Bower claimed, "We must all be grateful that Kate, in the end, I think, prevented Meghan from coming, and said she 'wouldn't have her there under any circumstances.' And if she [Markle] did come, she'd have to sit at the back."

The author also emphasized the importance of Prince Harry's presence at the event, stating, "I do think that for the Royal Family, Harry's presence is constitutionally important. God forbid, the Cambridges all died, we'd have King Harry on the throne after Charles, so he's got to be there."

Buckingham Palace confirmed on Tuesday that Prince Harry would travel to London to attend the May 6 coronation alone, while Markle will remain in California with the couple's two children, Prince Archie and Princess Lilibet.

Bower's assertion that Kate was the driving force behind the decision to exclude Markle from the event follows revelations about their strained relationship in Harry's memoir, "Spare." In the book, Harry claims that a passive-aggressive text argument initiated by Kate left Markle "sobbing on the floor" four days before their 2018 wedding. The disagreement was allegedly over Princess Charlotte's bridesmaid dress not fitting properly.

Tensions between the two women were also apparent in last year's Netflix docuseries "Harry & Meghan," in which Markle described her first meeting with Kate and Prince William while wearing "ripped jeans and [being] barefoot." Markle said, "Like, I was a hugger, always been a hugger. I didn't realize that that is really jarring for a lot of Brits," implying a chilly reception from the future king and queen.

However, Kate's camp responded to Markle's comments, with an unidentified friend telling People magazine, "Kate's a big hugger. She is warm and friendly and greets everyone with a big hug and kiss. It comes naturally to her to be like that."

The allegations by Bower follow earlier reports from an anonymous royal source that Kate was "relieved" Markle would not be attending the coronation. The Daily Mail quoted an unidentified royal observer who said the event "would have been particularly uncomfortable for Kate," as all eyes would be on the sisters-in-law due to their publicly disclosed disagreements.