Earl Spencer alleges King Charles reacted with 'elation' to Princess Diana's death

Princess Diana's brother has accused King Charles III of sounding "giddily elated, like a lottery winner" on the night his ex-wife was killed in a car crash almost three decades ago.

The explosive claim came in Charles Spencer's soon-to-be-released memoir, Swan Song: Diana, My Sister, an excerpt of which Britain's Daily Mail published on Thursday, local time.

Among several complaints about the royal family's behaviour at the time of his sister's death, Earl Spencer gave his account of the night of the 1997 crash.

"Unlike my other callers, who were stunned, barely able to articulate their condolences and offers of help, he [Charles] sounded giddily elated — like a lottery winner, I remember thinking at the time," the excerpt read.

"He let me know he had informed William and Harry of their mother's death, first thing that morning, in their bedrooms at Balmoral. It all sounded strangely matter-of-fact, given the enormity of the disaster.

"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. It can be hard to feel too much sympathy for an ex-spouse, or those who care for them profoundly, after bitter divorce."

The claims continue a war of words between Earl Spencer and Buckingham Palace that erupted earlier this week, when the first details of the book were made public.

Among several disparaging assertions, Earl Spencer alleged King Charles had told him "we'll forget her soon enough" in the days after Princess Diana was killed.

In response, 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 judgement and colour memory in ways others do not recognise, even many years after such a loss."

Royal commentators have described the rebuke as the most pointed statement they had seen from a British monarch in years.

Buckingham Palace is yet to respond to Earl Spencer's latest claims.

Princess Diana and King Charles — who was not on the throne at the time — divorced in 1996. Charles married Queen Camilla, with whom he had been having a long-running affair, in 2005.

The extract of Earl Spencer's book released on Thursday included several bombshell allegations, including that:

While Diana's deteriorating mental health, including a well-documented eating disorder, has been a matter of public knowledge for several years, Earl Spencer's direct and personal attacks linking them to King Charles are new.

The claims he makes in it have continued a devastating period for the royal family, which has included a public and ongoing rift between the king's two sons, William and Harry, as well as a police investigation into the monarch's brother, Andrew, who has been accused of child sex offences, which he denies.

In the past two years, Charles has also been treated for an undisclosed type of cancer, as has his daughter-in-law, Princess Catherine, who has recovered.

"It is only going to get worse; this is one extract that's being released from the book next week. One. I mean whatever else is in there is going to be explosive," royal commentator Afua Hagan told the ABC.

"Buckingham Palace is definitely going to be in damage control, and they have to be because Charles Spencer has always been so loyal to Princess Diana in her life and in her death, and he has really kept it in for a long time — all these things he has wanted to say."

Earl Spencer's book is expected to be released next week.