Prince Philip told Princess Diana her husband is not in 'right mind'

Prince Philip resisted King Charles over plea to break off his marriage with Princess Diana.

The Duke of Edinburgh wrote a letter to the former Princess of Wales in a bid to console the mother-of-two over Charles involvement with now-wife Camilla.

He wrote: “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.”

The letter was written in 1972, years before Diana and the King announced their separation.

Philip continued: “I cannot imagine anyone in their right mind would leave you for Camilla. Such a prospect never even entered our heads.”

He wrote: “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?”

"Fondest love, Pa", he signed off the letter.

It is reported that Diana often confided in Prince Philip over her marriage problems.