Royal author Christopher Andersen claimed that King Charles III now regrets having Princes William and Harry walk behind Princess Diana's coffin during her funeral procession in 1997 because now that memory haunts them.

During a conversation about his book "The King: The Life of Charles III" with US Weekly the biographer said, "I think it haunts him because it haunts them, and they've spoken about it."

He called it "a form of PTSD" and shared that while researching for his book, he learned that Prince Harry finds it "triggering" to fly to London sometimes.

Andersen said, "[He said] it reminds him of that day when he had to walk behind the coffin, and they were more or less bullied into doing it by the palace — by the men in gray who really run the palace, the people that Diana used to complain about."

He added that Princess Diana's brother Charles, Earl Spencer, "has also said that he felt that he was tricked into doing it and regrets it. He said it was like walking through a tunnel of grief."

"I think both William and Harry thought, 'Who are these strangers who never met her?' So they were angry about what had happened. And Charles, I think, understands that to some extent he was responsible for them having to suffer through [that]," Andersen continued.

Prince Harry has since spoken about how it was a difficult moment for him years later, in a June 2017 interview with Newsweek. He said, "My mother had just died, and I had to walk a long way behind her coffin, surrounded by thousands of people watching me while millions more did on television. I don't think any child should be asked to do that, under any circumstances. I don't think it would happen today."

Meanwhile, Prince William admitted to a mourner during Queen Elizabeth II's death in September that walking behind his grandmother's casket reminded him of his own mother's funeral. According to royal reporter Richard Palmer, the Prince of Wales confided that had been "very difficult."

Prince William and Prince Harry were only 15 and 12, respectively, when Princess Diana died in a car crash in Paris in August 1997. Aside from King Charles III and Earl Spencer, the late Prince Philip also walked with the brothers during the funeral procession because according to Andersen, he had promised to accompany them.

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6 September 1997: Prince Philip, Prince William, Earl Spencer, Prince Harry and Prince Charles walk outside Westminster Abbey during the funeral service for Diana, Princess of Wales Getty