Memoir by Princess Diana's brother to go on sale on September 22

LONDON - A memoir by the brother of Princess Diana detailing her life and troubled marriage to the now King Charles goes on sale on Tuesday, complete with the incendiary claim that the monarch was "giddily elated" at her death.

Charles Spencer, who delivered a much-lauded eulogy at the 1997 funeral of his sister, one of the most photographed women in the world, has sparked global headlines with a book that accuses the then-Prince of Wales of sounding like a "lottery winner" in the hours after her death.

Buckingham Palace hit back immediately, saying the king was "mindful ​that the pain of fraternal grief can cloud reason, judgment and memory", as it sought to prevent the king from being dragged back into one of the royal family's most damaging periods.

'AN EYEWITNESS TO HISTORY'

Spencer, 62, said he wrote the book to set the record straight and because it was his duty "as an eyewitness to history".

He said in the book that he spoke to ‌Charles ⁠shortly after Diana's death was confirmed. "Unlike my other callers, who were stunned, barely able to articulate their condolences and offers of help, he sounded giddily elated – like a lottery winner, I remember thinking at the time," he wrote, according to pre-released excerpts.

"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."

Diana was just 20 when she married the heir-to-the-throne Prince Charles in 1981, a fairytale wedding watched around the world. They had two children, William and Harry, but the relationship soon soured and, following years of briefing against one another in the media, they finally divorced in 1996.

That was a year after Diana told the BBC that "there were three of us in this marriage, so it was a bit crowded" in reference to Charles' relationship with Camilla Parker Bowles, whom he married in 2005.

A year later she was killed in a Paris car crash as her vehicle was pursued by photographers on motorbikes.

Spencer describes his book as an intimate, loving and honest portrait of his sister, who he said became a global icon overnight.

-Reporting by Kate Holton; Editing by Alex Richardson/Reuters