Prince Philip told Princess Diana he and the Queen "disapproved" of Charles's affair with Camilla Parker Bowles, a newspaper reports.
An inquest into Diana's death is yet to be held
The Daily Mirror reported that the Duke of Edinburgh had told the princess: "I cannot imagine anyone in their right
mind leaving you for Camilla."
The prince is said to have revealed his feelings on the matter in a letter to Diana in the summer of 1992, according to former butler Paul Burrell, whose book is being publicised
in the paper.
The Mirror said Prince Philip told Diana: "We do not
approve of either of you having lovers. Charles was silly to risk everything with Camilla for a man in his position.
"We never dreamed he might feel like leaving you for her. Such a prospect never even entered our heads."
But the newspaper also reported on Mr Burrell's allegation that the prince had written to Diana to ask her if she could "honestly look into her heart" and say Charles's affair with Mrs Parker Bowles was nothing to do with her behaviour towards him.
Prince Philip was also reported to have told Diana that her "irrational" behaviour after the
birth of Prince William had not helped her relationship with Charles.
The Mirror on Monday carried Mr Burrell's assertion that a letter from Diana had revealed she feared the brakes of her car were going to be tampered with, 10 months before she died in a crash in Paris.
The princess allegedly wrote in a letter to Paul Burrell: "This phase in my life is the most dangerous".
She reportedly named someone who was "planning an accident in my car, brake failure and serious head injury."
Diana and her lover Dodi Al Fayed were killed early on the morning of 31 August, 1997 when
a Mercedes driven by chauffeur Henri Paul crashed in the Pont D'Alma tunnel in
In the alleged letter, Princess Diana reportedly believed the plot was "in order to make the path clear for Charles to marry".
Mr Burrell was acquitted of theft in 2002. His trial collapsed after the intervention of the Queen.
He had been charged with stealing from Diana's estate.