A royal biographer has revealed the one regret King Charles has from Princess Diana’s funeral.

According to author Christopher Andersen, the monarch “deeply regrets” making his sons, Prince William and Prince Harry, join the procession behind their mother’s coffin.

“I think it haunts him because it haunts them, and they’ve spoken about it,” he told Us Weekly. “I’ve written that I believe it’s a form of PTSD.”

The writer of the new biography, The King: The Life of Charles III, added that while researching the book, he learned that the Prince Harry has found it “triggering” to fly into London at times.

“[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 grey who really run the palace, the people that Diana used to complain about,” he explained.

“[Charles, Earl Spencer], Diana’s brother … 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.”

At the time of their mother’s death, Prince William was 15 and Prince Harry was just 12-years-old.

Princess Diana died at age 36 after being chased by paparazzi in Paris, resulting in the fatal car crash in 1997.

“I think both William and Harry thought, ‘Who are these strangers who never met her?’” said Andersen. “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].”

Image credits: Getty Images 

This article first appeared on OverSixty.