Princess Diana’s brother, Charles Spencer, has accused King Charles of “gaslighting” him after Buckingham Palace pushed back at his claims about the monarch’s conduct.

Buckingham Palace released an unusually forceful statement Wednesday ahead of the release of a memoir by Spencer, which charts the breakdown of then-Prince Charles’s marriage to Princess Diana, and the fallout from her death in a Paris car crash in 1997, one year after the royal couple finalized their divorce.

In an initial excerpt released ahead of publication, Spencer recalled a heated phone call with Charles amid a disagreement about funeral arrangements, claiming that a furious Charles had told him Diana would “soon be forgotten.”

“While we do not comment on books as a matter of principle, His Majesty is mindful that the pain of fraternal grief can cloud reason, affect judgment and color memory in ways others do not recognize, even many years after such a loss,” the palace had responded.

In an interview with Britain’s Mail on Sunday newspaper, Spencer said: “It is not every day you wake up to find you’ve been gaslit by the king.”

“It 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,” Spencer added.

“Gaslighting” is a form of manipulation where the manipulator is trying to get someone to question their own reality, memory or perceptions.

Spencer also denied he was motivated by money in publishing the book, saying it “has never even been the remotest consideration” and stressing that “my primary responsibility as a historian is to tell the truth.”

The aristocrat, an earl whose family has generational ties with Britain’s royal family, has made a number of other allegations in excerpts since released from the book, published in advance by the Daily Mail newspaper.

In the excerpts, Spencer writes that Charles sounded “giddily elated” and “like a lottery winner” when they spoke by phone immediately after Diana’s death, adding: “I suspect the prince’s first reaction must have been to think more of how this event might free him to marry the woman he loved, rather than appreciating the loss of one he did not.”

He also speaks about his sister’s mental health and recounts conversations with her, recalling that she had told him how “things became very difficult indeed in the marriage early on.”

Spencer’s book, “Swan Song: Diana, My Sister,” is due to be released publicly on Tuesday.

Since its initial statement, Buckingham Palace has remained silent on the further allegations from Spencer’s book. NBC News attempted to seek further comment on Sunday, but the palace did not immediately respond.

There has been no public comment from Princes William and Harry, Charles and Diana’s two sons, on Spencer’s memoir.

Prince Harry, who quit as a working royal six years ago amid a reported rift with senior royals, has remained close to the Spencer family, with his own memoir “Spare” thanking “my mother’s siblings for their love, support, time, and perspective.”

In an interview with the BBC that is set to air Sunday, Spencer said that Harry and his wife Megan Markle’s treatment at the hands of Britain’s infamous tabloid press has “echoes” of what happened to Diana.

“If you look at an article in a tabloid about Diana or the couple that you were mentioning, Harry and Meghan, you know, they can be really, really unpleasant,” Spencer said when questioned whether he believed Diana would “identify” with the scrutiny faced by Harry and Megan.

He added: “I think it would be really beholden on people to learn from Diana’s example. I’m not here to speak for Harry and Meghan, but for anyone who’s being destroyed in this way daily, it must be such a cancerous influence in their life.”

The royal family continued to undertake public duties this week even as Spencer’s book has dominated the headlines, with Charles also meeting with senior figures in AI at a summit in Scotland.