Charles Spencer’s new book about his late sister, Princess Diana, goes on sale after searing extracts prompted a rare and sharp rebuttal from King Charles III.

In ‘Swan Song: Diana, My Sister’, Spencer says he wants to speak on Diana’s behalf and correct untruths that have become accepted over time. The memoir alleges rows with the then-Prince Charles over the 1997 funeral and accuses him of contempt for his ex-wife.

Spencer claims he argued with Charles over palace plans for Prince William and Prince Harry, then 15 and 12, to walk behind their mother’s coffin. He says the then-Prince Charles “went ballistic” and told him: “Rest assured, we’ll forget her soon enough.” He also says Charles sounded “elated” after Diana’s death in a Paris car crash at 36.

Buckingham Palace responded with a strongly worded statement, saying grief can cloud reason and colour memory.