Charles Spencer’s new book about his late sister, Princess Diana, includes an inside look at the aftermath of her death and funeral.

The Daily Mail published the first excerpt of the 9th Earl Spencer’s upcoming memoir, Swan Song: Diana, My Sister, on Thursday, Sept. 17, including his account of running point between his family, the Buckingham Palace faction led by Queen Elizabeth and Diana’s ex-husband, then future King Charles, as they planned her funeral.

“Diana remained theirs, in Buckingham Palace’s eyes, even in death,” he wrote. “Her family, despite her divorce, were simply expected to fall into line with any royal decisions.”

Spencer said he tried to make requests on the family’s behalf: “My sisters knew that Diana had always wanted Mozart’s Requiem to be played for her, after her death.”

“But when I passed this on, it was rejected as a possibility: ‘No part of a requiem can be played at a Church of England funeral,’ I was told,” he recounted. “I later found out that was not the case — the first of several untruths that the authorities would push my way at that time.”

Spencer wrote in the book that, while some in the royal inner circle treated him with kindness, “There were others in the Palace, I already knew, who were relieved — ­indeed, pleased — that she had gone.”

He also recounted his feud with the royals over whether or not Diana’s sons, Prince William and Prince Harry, then ages 15 and 12, would walk behind her coffin during the funeral procession.

“ ‘Well, that’s not going to happen,’ I said, in a ­matter-of-­fact tone,” Spencer recalled. “It was a suggestion that struck me as utterly barbaric, and quite out of the question.”

Diana had left Spencer as her sons’ guardian, and he says he protested that she “would never have wanted to put them through anything so ghastly.”

But it was their father, then-Prince Charles, who was still in charge. The royal would later call Spencer over the disagreement.

“ ‘I gather you’ve said you don’t want William and Harry to walk behind Diana’s coffin,’ ” Spencer recalls his former brother-in-law saying, to which he replied, “Well, of course not, sir. Diana would never have wanted that.”

When Prince Charles said he did it at the funeral for his uncle Lord Mountbatten, Spencer claimed to have told him, “ ‘But you were 30, sir, and he wasn’t your mother.’ ”

“He went nuts,” Spencer wrote, per the excerpt. “ ‘That’s the problem with your family. They’ve got no bloody sense of duty!’ he started, before descending into a general rage.”

Spencer continued, “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!’ I was so stunned that it took me a few seconds to reply. ‘I can’t believe you just said that,’ I said flatly, before hanging up.”

PEOPLE reached out to Buckingham Palace, which declined to comment. One day prior, the palace issued a rare statement after the passage about King Charles’ comment that they would “forget her soon enough” was first previewed by the Daily Mail.

The statement from a palace 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 color memory in ways others do not recognize, even many years after such a loss.”

Both Prince William, now 44, and Prince Harry, now 42, confirmed in the BBC’s 2017 documentary Diana, 7 Days that it was a “group decision” for them to join their mother’s funeral procession, with William saying, “It wasn’t an easy decision, and it was a collective family decision to do that.”

Harry wrote about his uncle’s defense of him and his brother in his 2023 memoir, Spare.

“It seemed a lot to ask of two young boys. Several adults were aghast. Mummy’s brother, Uncle Charles, raised hell. ‘You can’t make these boys walk behind their mother’s coffin! It’s barbaric.’ “

An alternative plan was proposed: Prince William would walk alone. “He was 15, after all,” Harry wrote. “ ‘Leave the younger one out of it.’ Spare the Spare.”

But the answer came back: “It must be both princes. To garner sympathy, presumably.”

“Uncle Charles was furious,” Harry wrote. “But I wasn’t. I didn’t want Willy to undergo an ordeal like that without me. Had the roles been reversed, he’d never have wanted me — indeed, allowed me — to go it alone.”

Swan Song: Diana, My Sister was announced last month. The book marks the first time Spencer speaks “openly” and “at length” about his late sister, and he said he was inspired to write it after being “inundated” with requests for interviews as the 30th anniversary of Diana’s death approaches.

After recording the audiobook, Spencer recounted the experience in an Instagram media post, calling Swan Song the “most personal” book he’s ever written.

“At heart, this is a brother’s simple tribute to an extraordinary sister,” he wrote. “When I finished the recording, I thought: ‘That is exactly what I wanted and needed to say — and she would, I reckon, have approved of every word.’ ”