By Issy Ronald

(CNN) — Only a few extracts from a new book by Princess Diana’s brother, Earl Charles Spencer, have published so far. But they have sparked a firestorm for Britain’s royal family, dredging up painful memories of Diana’s unhappy marriage to King Charles III, and provoking a rare, forceful rebuke from Buckingham Palace.

“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,” a palace spokesperson said in a statement.

CNN has not seen the book “Swan Song” ahead of its publication, but a spokesperson for Penguin Random House has confirmed the passages quoted by the Daily Mail, which is exclusively publishing excerpts from the book, are accurate.

These extracts contain several revelations about Charles and Diana’s marriage, as well as the then-Prince of Wales’ reaction to her death.

Here’s what you need to know about the book before its release next week:

Charles ‘giddily elated’ after Diana’s death

Perhaps the most incendiary allegations in these extracts are Spencer’s memories of how Charles reacted to Diana’s death.

He details that fraught night on August 31, 1997, tracing the moment from when news first emerged of Diana’s fatal car crash in Paris, his frequent phone calls with his sister Jane, to when he rang his former brother-in-law after learning Diana had died.

In that call, Spencer says Charles “sounded giddily elated – like a lottery winner.”

“Looking back now, I suspect the prince’s first reaction must have been to think more of how this event might free him to marry the woman he loved, rather than appreciating the loss of one he did not,” Spencer claimed.

‘We’ll forget her soon enough’

During a later call between the two men, as they argued whether Princes William and Harry should walk behind their mother’s coffin at her funeral, Spencer says Charles unleashed “a stream of anger,” before briefly stopping.

“I assumed the pause signaled 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,” Spencer wrote.

“‘Rest assured,’ he hissed, ‘we’ll forget her soon enough!’” Spencer wrote. “‘I can’t believe you just said that,’ I said flatly, before hanging up.”

Charles and Diana’s unhappy marriage

Spencer also details his perception of his sister’s unhappy relationship with the then-Prince of Wales, and even claims that Diana tried to break off the engagement after Charles had “shown next to no interest in her” at his brother Andrew Mountbatten-Windsor’s 21st birthday party.

“Immediately after that evening of escalating heartbreak, Diana went to see my father,” Spencer wrote. “She told him she could not go through with marriage to a man who clearly did not love her.” Her father refused to allow Diana to break the engagement, Spencer said.

That incident came shortly after Diana had found a bracelet Charles bought for his now-wife Queen Camilla, Spencer claimed.

And on the day before their wedding, Charles told Diana “she needed to know that he was very happy to marry her, but he was not actually in love with her,” Spencer wrote, citing a story his sister had told a “close friend.”

‘Oh, a bit chubby here, aren’t we?’

Just days after the couple’s engagement announcement, Spencer claims Charles squeezed Diana’s waist and said, “Oh, a bit chubby here, aren’t we?”

Diana, who suffered from bulimia, found that comment “devastating,” Spencer said.

“She would later share with me that her waist shrank from 29 to 23½ inches between her engagement and her marriage,” Spencer wrote.

Queen Elizabeth II offered to restore Diana’s HRH title

On the day of Diana’s funeral, Queen Elizabeth II offered to restore the “HRH” title she had lost following her divorce to Charles, Spencer claimed.

The letters, which stand for “His or Her Royal Highness,” are used to denote specific senior royals. The late Queen had declared a year before the tragedy that the HRH style would be taken away in the event of a divorce – as was the case for Diana and Sarah Ferguson following her divorce from then-Prince Andrew.

Spencer said Diana was “dreadfully hurt” by the “petty and vindictive” removal of that title, and he decided to reject Elizabeth’s “well-meaning” offer as a “consolation prize.”

“I felt that I knew in my bones how she would view this posthumous award,” he wrote, recalling a conversation he had with the Queen’s private secretary, Robert Fellowes. “That’s very kind of the Queen, of course. But what’s the point, Robert? Now? I mean – Diana’s dead.”

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