Earl Spencer, the brother of Diana, Princess of Wales, has claimed King Charles said “we’ll forget her soon enough” days after her death, in an extract from his upcoming memoir published by the Daily Mail.

He claimed the then-Prince of Wales subjected him to a “stream of anger” after he insisted Diana would not have wanted Princes William and Harry, then aged 15 and 12, to have to walk in her funeral procession.

Buckingham Palace has strongly pushed back against the claim.

A spokesperson said: “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.”

Swan Song: Diana, My Sister is due to be published next week and is expected to be highly critical of the Royal Family. BBC News has not seen the book in full.

In the first extract, external from it to be made public, Earl Spencer claims to have been informed by a senior Palace aide that the young princes would walk alongside Princess Diana’s coffin, which he objected to.

He claims to have received a phone call from Charles shortly after.

Earl Spencer wrote in the book: “He [Charles] 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’.”

Earl Spencer’s book about his sister seems set to open a very difficult time in the recent history of the Royal Family.

The death of Diana, a hugely popular figure, in a car crash in 1997, had created a volatile atmosphere, in which the royals were accused of failing to show feeling and empathy, and had seemed out of touch with the public mood.

Earl Spencer’s emotional and defiant speech at her funeral added to that cloud of grievance hanging over Diana’s untimely death.

Prince Harry has previously said that walking behind his mother’s coffin aged 12 was something no child “should be asked to do”.

In 2017, he told the BBC he didn’t “have an opinion whether that was right or wrong”, but “looking back on it”, he was glad to have been part of the day.

Earlier on Wednesday, it emerged that copies of Earl Spencer’s unreleased book had been stolen from a truck in the Netherlands.

BBC News understands four copies of the Dutch language edition were taken from a truck while it was en route from the Dutch publisher’s printers to their warehouse.