PAUL BURRELL: BOOK OF THE CENTURY: I can't imagine anyone in their right mind leaving you for Camilla; PHILIP'S AMAZING LETTERS TO DIANA.
PRINCE Philip told Princess Diana he and the Queen disapproved of Charles's affair with Camilla Parker Bowles.
He even said: "I cannot imagine anyone in their right mind leaving you for Camilla."
The sensational confession came in a letter to Diana, revealed for the first time in Paul Burrell's book A Royal Duty.
And it dispels the myth that he disliked Diana and blamed her for damaging the monarchy.
Philip's letter was sent to the princess in the summer of 1992, when her marriage was collapsing over Charles's adultery.
The duke 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 in other correspondence with Diana - details of which are also in Burrell's book - Philip tried to blame her for the split.
He asked her if she could "honestly look into her heart" and say Charles's affair was nothing to do with her behaviour towards him.
The revelations come after the Mirror yesterday published an explosive letter from Diana to former butler Burrell in which she told of a plot to kill her in a car crash so Charles could remarry.
Britain reacted in shock to the news the princess was convinced she would die 10 months before her fatal smash in a Paris tunnel.
Burrell's book gives a fascinating insight into Philip and the Queen's private thoughts about their son and his mistress.
Today he also reveals details of the heartbreaking events at the Paris hospital after he was sent to collect Diana's body.
And he tells how the duke claimed jealousy was the cancer in the princess's marriage.
Philip also warned Diana after the birth of Prince William, that her "irrational" post-natal behaviour did not help her relationship with Charles.
And he told her being the prince's wife "involved much more than simply being a hero with the British people".
But despite those exchanges, Burrell insists Diana's relationship with the duke was not the frosty one many believed existed.
He denies claims Philip described the princess as a harlot or trollop in his letters.
In fact, says the former butler, Diana had enormous respect for her father-in-law's honesty.
Burrell tells how Diana "leapt around the room with unbridled joy" at Philip's letter on Camilla. He signed some "fondest love, Pa."
His book tells of a meeting between Charles, Diana, the Queen and Philip at the height of the scandal over Morton's book.
The princess later told Burrell: "Mama despaired as she listened to me. I think she aged at that time because all I seemed to be doing was relaying to her my anguish."
Philip's letters began in earnest after she failed to turn up to a second meeting and continued until the following autumn.
CONCERN: Diana with Philip; CONFESSION: Duke was not happy at affair; SUPPORT: Diana with Paul Burrell