King Charles III allegedly told Princess Diana’s brother that she would be forgotten “soon enough” in a horrific phone call following her death in 1997.

Charles Spencer, the 9th Earl Spencer, details in his forthcoming memoir “Swan Song: Diana, My Sister,” that the monarch allegedly released his “pent-up contempt for Diana” after the then-Princess of Wales died in a Paris car crash.

“He unleashed down the phone what I have always taken to be his pent-up contempt for Diana, and his horror that the world was so bowled over by her death,” Spencer claimed, per an excerpt obtained by the Daily Mail. “’Rest assured,’” he hissed, ‘we’ll forget her soon enough!’”

King Charles III allegedly told his late ex-wife Princess Diana’s brother that she would be forgotten “soon enough” shortly after she died. (Seen above: Prince Charles, Princess Diana, Prince William, and Prince Harry in 1986.) Getty Images

Charles Spencer, the 9th Earl Spencer (seen here), wrote about the phone call in his upcoming memoir “Swan Song: Diana, My Sister.” Getty Images

Spencer recalled being too “stunned” to respond to Charles’ alleged remarks.

“I can’t believe you just said that,” Spencer told Charles, now 77, before hanging up.

A rep for Buckingham Palace told Page Six, “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 colour memory in ways others do not recognise, even many years after such a loss.”

Spencer described the monarch’s alleged remarks as “pent-up contempt for Diana.” (Seen here: Charles arriving at a gathering in Scotland on Sept. 5.) AFP via Getty Images

“He unleashed down the phone what I have always taken to be his pent-up contempt for Diana, and his horror that the world was so bowled over by her death,” Spencer alleged. (Seen here: Diana in January 1988.) AFP via Getty Images

According to the outlet, Spencer’s book will also dive into Diana’s marriage and 1981 fairytale wedding at St. Paul’s Cathedral in London.

The author also explores the exes’ failed marriage, including extramarital affairs leading up to their tumultuous divorce in 1996, one year before she died at the age of 36.

Diana’s vehicle crashed inside Paris’ Pont de l’Alma tunnel after being chased by paparazzi. Diana, along with her boyfriend, Dodi Fayed, and driver Henri Paul, also died.

“’Rest assured,’” he hissed, ‘we’ll forget her soon enough!’” Spencer recalled Charles allegedly saying. (Seen here: Prince Harry, Prince William and Dr. Andrew Gailey with Diana and Charles in 1995.) Getty Images

“I can’t believe you just said that,” Spencer (seen here with Diana) allegedly told Charles. PA Images via Getty Images

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Spencer also wrote about Diana’s funeral at London’s Westminster Abbey on Sept. 6, 1997, and how he allegedly received a call days earlier from Queen Elizabeth’s aide, insisting that Diana and Charles’ sons, Prince William and Prince Harry, wouldn’t be participating in the procession.

“That’s not going to happen,” Spencer allegedly told the aide.

Sure enough, William and Harry, who were 15 and 12, respectively, joined Spencer, along with Charles and Prince Philip, during the funeral procession.

Diana (seen here in 1994) died in a car crash in 1997. WireImage

Spencer also wrote about her funeral. (Seen above L-R: Prince Philip, Charles, William, Harry and Spencer at Diana’s funeral.) Getty Images

Spencer’s tell-all memoir will be released on Sept. 22.

Ahead of the book’s release, four copies were reportedly stolen from a truck heading to a warehouse for distribution in the Netherlands.

The driver had parked overnight and woke up to a break-in.