Buckingham Palace has hit back at claims by the late Princess Diana's brother that King Charles III acted insensitively following her death in a car crash almost 30 years ago.

The palace issued an unusually strong statement after Charles Spencer claimed in his new memoir that the then-prince, Diana's former husband, said "rest assured, we'll forget her soon enough" days after she died in 1997.

"While we do not comment on books as a matter of principle," a palace spokesperson said late Wednesday.

"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."

"Swan Song: Diana, My Sister" is due to be published next week.

In the book, serialised in the Daily Mail, Spencer claims that Charles subjected him to a "stream of anger" after he insisted Diana would not have wanted her sons, princes William and Harry, then aged 15 and 12, to walk in her funeral procession.

"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. 'Rest assured', he hissed, 'we'll forget her soon enough!'," Spencer writes.

Diana, who was extremely popular in the UK, died in Paris at the age of 36 after the car she was travelling in crashed as it tried to get away from paparazzi photographers.

Her death sparked an outpouring of grief in Britain and led to accusations that the monarchy failed to show the appropriate empathy.

UK media likened the language in the palace's statement to the late Queen Elizabeth II's response in 2021 to claims of racism in the royal family from Harry and his wife Meghan.

The queen said that "some recollections may vary".