The allegation appears in a memoir by her brother, Earl Spencer which is being serialised by the Daily Mail.

According to an extract from Swan Song: Diana, My Sister, the then Prince Charles made the alleged comment during an argument about arrangements for her funeral after she died in a car crash in Paris at the age of 36 in August, 1997.

The disagreement concerned whether Diana’s sons, Prince William and Prince Harry, should walk behind their mother’s coffin during the procession. William was 15 and Harry was 12 at the time.

Prince Charles (now King Charles III), Earl Spencer, Prince Harry and Prince William walk behind the funeral cortege of Diana, the Princess of Wales at her funeral at Westminster Abbey, London (Image: Adam Butler/PA Wire)

Earl Spencer claims that one of Queen Elizabeth II’s senior aides told him the boys were expected to join the procession. He says he replied: “That’s not going to happen.”

The Earl says Charles called him minutes later and referred to having walked in the funeral procession of his great-uncle, Lord Mountbatten, in 1979.

Earl Spencer claims he pointed out that Charles had been 30 when he took part in that procession.

In the book, Earl Spencer said: “Prince Charles continued his stream of anger, without me interrupting. Then he stopped.”

Earl Spencer addresses the congregation in front of the........

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