King Charles has hit out at Earl Spencer over claims made in his former brother-in-law's new book about the late Princess Diana.

Buckingham Palace suggested the 62-year-old earl's grief over his beloved sister - who died in a Paris car crash in August 1997 - had "clouded reason" and "affected judgement" after he claimed in Swan Song: Diana, My Sister that the monarch had sneered Diana would be forgotten "soon enough" during a phone call in the days following her passing.

In a very rare statement, a spokesperson for the king 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."

In his book, the former Today correspondent alleged he and Charles, then Prince of Wales, argued when debating whether Princes William and Harry, who were 15 and 12 respectively, should walk behind their mother's coffin during her funeral procession.

After the royal told his brother-in-law he had walked in the funeral procession of his beloved great uncle Lord Mountbatten in 1979, the earl pointed out he had been 30 at the time and claimed the prince "went nuts".

In a preview from the clip serialised by the Daily Mail, the earl wrote: "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!'"

A source close to the king insisted to the Telegraph that the alleged remark was "just not in his nature and it certainly wasn't in his nature at that time, when he was a man absolutely grief-stricken".

They noted Charles was "paralysed with grief" for his ex-wife and had defied protocol by insisting the princess be given a full state-style ceremonial funeral, and also that her coffin be draped in the royal standard as it returned from Paris to London.

Swan Song: Diana, My Sister chronicles the princess' childhood, her difficult marriage to Charles and the aftermath of her death. It is scheduled for publication on 22 September.