The background to the letters was that while the Queen would listen to Diana’s woes, she would do little about it, hoping the problems would solve themselves. She would then leave it up to Philip to put his persuasive powers to good use and try to reason with her. He confided in his daughter-in-law, writing to her and explaining that he knew first-hand the difficulties of marrying into the royal family. He tried to make her face facts and deal with the problems within her marriage, and not put her head in the sand. He signed them ‘Pa’, and in the beginning was sympathetic.

The correspondence between them began in earnest just after the Andrew Morton book was published in June 1992. This provoked a summit meeting at Windsor Castle, with Charles and Diana discussing the state of their marriage with the Queen and Prince Philip. Diana believed the only solution was a separation, but the Queen hoped a compromise would be possible, for the sake of the children and the monarchy. When Diana failed to appear at a second meeting, Prince Philip decided to take matters into his own hands.

He was direct and straight to the point, but not unkind – or at least he didn’t think so. One of his fiercest comments was: ‘Can you honestly look into your heart and say that Charles’s relationship with Camilla had nothing to do with your behaviour towards him in your marriage?’

Philip also told Diana she had not been a caring wife and that while she was a good mother, she had been too possessive with her sons. Like most insecure people, Diana did not appreciate being told off one bit and furiously showed the letters to several of her friends, including Rosa Monckton and Lucia Flecha de Lima, who helped her construct suitable replies. Another letter, which according to Burrell had her leaping with joy, said: ‘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. I cannot imagine anyone in their right mind leaving you for Camilla. Such a prospect never even entered our heads.’

He also told Diana that jealousy had eaten away at the marriage and that her irrational behaviour after the birth of William had not helped. He added that her husband had made a ‘considerable sacrifice’ by cutting ties with Camilla and that Diana had not ‘appreciated what he had done’. He even went on to suggest Diana’s behaviour had driven Charles into Camilla’s arms. In one letter, he told her being the wife of the heir to the throne ‘involved much more than simply being a hero with the British people’.

He did, however, start one letter with the words: ‘Phew!!! I thought I might have gone a bit too far with that last letter.’