The longstanding tension between Kate Middleton and Meghan Markle reportedly intensified following Queen Elizabeth's passing in September, with one royal insider claiming that the Duchess of Cambridge harbors resentment towards her sister-in-law.

In his new book, "Our King: Charles III: The Man and the Monarch Revealed," royal biographer Robert Jobson alleges that Middleton, 41, missed the opportunity to say her final farewells to Queen Elizabeth due to efforts to prevent Prince Harry, 38, and Meghan Markle, 41, from traveling to Scotland. According to Jobson, King Charles ordered both spouses to remain in England, deeming it "not appropriate" for them to be at the Queen's bedside.

"Harry was insisting Meghan travel with him to Scotland as the queen's life ebbed away, but the king said it was only for the children and grandchildren to be with the queen," Jobson claimed. The biographer also stated that Charles, 74, asked Prince William not to bring Middleton along, reasoning that "if Catherine doesn't come, Meghan can't either."

Jobson explained, "Privately, he wanted to say Meghan was not welcome — but he couldn't say that to Harry, so he personally intervened and asked Kate to stay back so that it was fairer on Meghan." However, Middleton was eager to visit Balmoral to see the monarch one final time due to their close relationship.

"Kate deliberately stayed away, but she desperately wanted to be there with the Queen in her last moments," Robson wrote. "That's eaten Kate up and has built up resentment towards Meghan."

Following Queen Elizabeth's death on September 8, several members of the royal family traveled to Scotland, including Prince Charles, Camilla, Princess Anne, and the Queen's physician, Dr. Huw Thomas. Prince William, Prince Edward, Sophie, and Prince Andrew also journeyed to the royal residence after the monarch's passing.

In his book, Jobson also recounted how Middleton found the walkabout she and William participated in with Harry and Markle days before the Queen's funeral as "one of the hardest things" she ever had to do. The walk at Windsor Castle was suggested by Prince William as a show of unity in the wake of Elizabeth's passing but was reportedly delayed by the Duke and Duchess of Sussex at the time.