The feud between sisters-in-law Kate Middleton and Meghan Markle apparently reached a boiling point after the passing of Queen Elizabeth last September, according to royal biographer Robert Jobson’s new tell-all book, “Our King: Charles III: The Man and the Monarch Revealed.”

Jobson alleges that Middleton harbors resentment towards Markle for preventing her from saying goodbye to the queen.

The Princess of Wales has “built up resentment” toward Markle for making the former miss saying goodbye to her beloved monarch.

Middleton, 41, allegedly felt she was forced to stay back in England as Elizabeth died at Balmoral — in an attempt to prevent Prince Harry, 38, and Markle, 41, from traveling to Scotland.

Jobson claimed that King Charles had ordered both wives to stay behind as it was “not appropriate” for spouses to be by the queen’s deathbed.

“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, 49, claimed.

The biographer also alleged that Charles, 74, asked Prince William not to bring the then-Duchess of Cambridge as, “if Catherine doesn’t come, Meghan can’t either.”

Jobson added, “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 anxious to jet-set off to Balmoral to see the monarch one last time before she died due to her close friendship with Queen Elizabeth.

“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.”

Elizabeth died on Sept. 8, and Middleton was spotted that day picking up her children Prince George and Princess Charlotte from school.

Members of the royal family who had rushed to Scotland to see the sovereign were Charles, his wife Camilla, Princess Anne and the queen’s doctor, Huw Thomas.

William, 40, as well as Prince Edward, his wife Sophie and Prince Andrew also flew to the Firm’s Scottish residence following the queen’s death.

Harry, had also arrived, albeit a little too late to say goodbye to his grandmother.

The Post has reached out to palace reps as well as Prince Harry and Markle for comment on the new allegations.

Meanwhile, elsewhere in Jobson’s book, he scribed how Middleton found the walkabout she and William did with Harry and Markle just days before the queen’s funeral as “one of the hardest things” she ever had to do.

On Sept. 10, the group — dubbed “the Fab Four” by the snarky UK press — walked the grounds of Windsor Castle to greet well-wishers and mourners.

He explained that the outing was an idea suggested by William as a way to present a united and joint front amid Elizabeth’s passing.

But the stroll was allegedly delayed by the Duke and Duchess of Sussex at the time, NBC’s royal commentator Daisy McAndrew claimed.

“Interestingly, the reason that the walkabout was so late — it was scheduled to be nearly an hour before they actually came — was because, of course, Harry and Meghan had to go and get changed,” McAndrew told the “Today” show on Sept. 13, adding that they were about an hour late for the event.

“They weren’t wearing the proper formal mourning attire, so they had to quickly go and get changed — I guess, do hair and makeup for Meghan. So that’s why they were late.”