A furious Prince Charles told Earl Spencer days after Princess Diana’s death, ‘rest assured, we’ll forget her soon enough’, according to a devastating memoir.

In allegations that will send shockwaves through the Royal Family, Charles Spencer’s bombshell book – to be exclusively serialised in the Daily Mail – lifts the lid on an incendiary row with the now King over funeral arrangements.

The argument followed the decision, opposed by Diana's brother the Earl, that Prince William and Prince Harry, then aged 15 and 12, would walk in their mother’s funeral procession.

The Earl says he insisted to the then Prince of Wales that Diana would not have wanted her young sons to endure such a public ordeal. It is claimed Charles then questioned the Spencer family’s sense of duty before saying his ex-wife, whose death sparked a global outpouring of grief, would soon be forgotten.

Earl Spencer says the comments ‘unleashed’ by the King almost 30 years ago reflected ‘his pent-up contempt for Diana, and his horror that the world was so bowled over by her death’.

The astonishing account of the King’s alleged outburst is included in Swan Song: Diana, My Sister, which Earl Spencer, 62, says he has written to correct ‘untruths’ told about his older sister.

The Daily Mail can disclose that it includes a string of further extraordinary claims about Diana’s failed marriage to King Charles and the tumultuous aftermath of her tragic death in a car crash in Paris in 1997.

Revelations from the book are set to plunge Buckingham Palace into a fresh storm just weeks after the dramatic return of the Duke and Duchess of Sussex and their children to the UK from California.

On September 6, 1997, six days after Diana’s death, Earl Spencer famously delivered a defiant eulogy at her funeral that was later named one of the greatest speeches of the 20th century.

His raw oration in Westminster Abbey was delivered after he walked alongside William and Harry, as well as Prince Charles and Prince Philip, behind Diana’s coffin in a funeral procession through central London broadcast to 2.5billion people worldwide.

In his book, Earl Spencer writes that when one of Queen Elizabeth’s most senior aides told him in the days before the funeral that William and Harry would take part in the procession, he responded: ‘That’s not going to happen.’

But minutes later he received a phone call from Prince Charles who insisted that 18 years earlier he had walked in the funeral procession of Lord Mountbatten, his great-uncle, who he affectionately called ‘uncle Dickie’.

The memoir describes how Charles ‘went nuts’ after Earl Spencer pointed out that the Prince had been 30 at the time of Lord Mountbatten’s funeral in 1979.

In one of the book’s most extraordinary passages, Earl Spencer writes: ‘Prince Charles continued his stream of anger, without me interrupting. Then he stopped.

‘I assumed the pause signalled an effort to bring himself under control, but then 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!”’

Earl Spencer said that he was ‘so stunned’ by the alleged remarks that he took a few seconds to reply.

‘“I can’t believe you just said that,” I said flatly, before hanging up,’ he writes.

Earl Spencer’s description of the shocking outburst threatens to reopen the wounds of the so-called War of the Waleses and Prince Charles's bitter split and eventual divorce from Diana.

Described as ‘historic’ and ‘extraordinarily moving’, Swan Song is due to be published worldwide next Tuesday – in the UK by Penguin Michael Joseph, a division of Penguin Random House.

It comes as police in the Netherlands investigate what publishing sources say is the ‘extremely suspicious’ theft of four copies of the memoir from a lorry earlier this week.

The Daily Mail revealed how a lorry packed with thousands of copies of the book, which were wrapped in black plastic to hide their contents, was broken into after it was parked overnight in a car park while the driver slept in the cab.