Buckingham Palace has issued an unusual public statement after Earl Spencer alleged King Charles told him “we’ll forget her soon enough” in the days following the death of Diana, the late Princess of Wales.

The claim appears in Earl Spencer’s forthcoming memoir, Swan Song: Diana, My Sister, which is due to be published in the UK on Tuesday. An extract from the book was published by the Daily Mail on Wednesday, prompting the palace response.

In its statement, Buckingham Palace 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.”

According to Spencer, a palace aide told him that Diana’s sons, Prince William and Prince Harry, then aged 15 and 12, would walk behind her coffin during the funeral procession. He said he objected because he believed Diana would not have wanted that. In the memoir, he claims the then-Prince of Wales reacted furiously.

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’.”

The book is said to be the first time Earl Spencer has spoken at length about his sister’s death, according to its publisher, Penguin Michael Joseph, a division of Penguin Random House UK. It is expected to explore Diana’s childhood, her unhappy marriage to the then-Prince of Wales, and the aftermath of her death in Paris. It will also examine how the royal family responded to the vast public grief and anger that followed.

Diana died in August 1997 in a car crash in Paris alongside her partner, Dodi Fayed, and their chauffeur Henri Paul. At her funeral later that year, Earl Spencer walked behind the coffin with Prince William, Prince Harry, then-Prince Charles, and the Duke of Edinburgh.

The question of whether Diana’s sons should take part in that procession has surfaced before. In Prince Harry’s 2023 memoir Spare, he wrote that there was a heated argument over the issue. Harry said Earl Spencer “flew into a rage” saying: “You cannot force these children to walk behind their mother’s coffin. It’s a barbarity!”

Announcing the memoir last month, Earl Spencer said he had been “inundated yet again” with interview requests about Diana as the 30th anniversary of her death approaches in 2027. “This has convinced me to write down my own thoughts and memories, once and for all, rather than having the discomfort of again reading the opinions of others, many of which are based on untruths that have become accepted over time,” Earl Spencer, 62, said.

“The aim of this book is to tell the truth about Diana from her brother’s perspective, just as I attempted to do when speaking at her funeral,” he said, referring to the eulogy he delivered in Westminster Abbey in 1997. “As with my eulogy, I want to speak on Diana’s behalf and do so in an openly positive way.”

In that speech, Spencer sharply criticised the media and described his sister as the “most hunted person of the modern age”. He also vowed to look after her sons. “I pledge that we, your blood family, will do all we can to continue the imaginative way in which you were steering these two exceptional young men so that their souls are not simply immersed by duty and tradition but can sing openly as you planned,” he said at the time.

Spencer has also said he wants the book to reach readers too young to remember Diana when she was alive. “I hope this book will be of interest to the many who still cherish the memory of Diana when she was alive, and I would be delighted if those too young to remember her in her heyday feel − after reading this − that they ‘get’ who she really was,” he said. “A one-off dynamic character, full of love, charisma and self-doubt, who gave life a hell of a go, and left the world a better place, despite exiting the stage while still so young.”

In July, Prince Harry visited Diana’s burial site at Althorp in Northamptonshire with his wife Meghan and their two children, Prince Archie and Princess Lilibet.

Ahead of publication, the book was also caught up in a separate incident. Earlier on Wednesday, four copies were stolen from a truck in the Netherlands after the Dutch language editions were taken while the driver slept at an overnight rest area. The vehicle had left a printing plant in central Europe on Monday and was travelling from the Dutch publisher’s printers to its warehouse.

Swan Song: Diana, My Sister is being released next week, ahead of the 30th anniversary of the royal’s death in Paris next year.