Charles Spencer, the brother of Princess Diana, on Sunday accused King Charles III of "gaslighting" him, a term for making someone doubt their own memory or perception, by questioning his explosive account of the events surrounding her death in an upcoming memoir.
Spencer, whose book about his older sister, "Swan Song", comes out Tuesday, told Britain's Mail on Sunday newspaper: "It is not every day you wake up to find you've been gaslit by the King", escalating a war of words after Buckingham Palace, the British monarch's official London residence and office, issued a rare statement.
A Palace spokesperson reacted to the memoir's claims Thursday, saying the 77-year-old king "is mindful that the pain of fraternal grief can cloud reason, affect judgment and color memory in ways others do not recognise, even many years after such a loss".
This came after a sensational claim in memoir excerpts serialized in the Daily Mail, a British tabloid newspaper, that Charles, then Prince of Wales, told Spencer in a phone call shortly after Diana's death in 1997: "Rest assured, we'll forget her soon enough."
Spencer claimed Charles "sounded dizzily elated, like a lottery winner".
The 62-year-old aristocrat told the Mail on Sunday the royal statement "was astonishingly provocative and below the belt because it was essentially trying to say I'd lied without the King being able to deny what I'd said".
He also dubbed it a "rather poor imitation" of the Palace's response to Prince Harry and his wife Meghan's claims of royal racism in an interview with US talk show host Oprah Winfrey five years ago, which was that "recollections may vary".
Spencer formerly worked as a US television journalist and has written books about history and his school days. He inherited the family estate where Diana is buried.
After her death, he accused the media of having "blood on their hands" and, in a eulogy at her funeral, vowed always to look out for her two sons, William and Harry.