Mr Kay said the Princess had confided in him about a bitter fight she had with the Queen's heir, and admitted Charles had asked her: "Do you seriously expect me to be the first Prince of Wales and not to have a mistress?"
Diana also opened up about the affair and it's effect on her to journalist Andrew Morton for her 1992 biography "Diana: Her True Story".
The mother-of-two admitted she confronted Camilla, saying: "I know what's going on between you and Charles and I just want you to know that."
Princess Diana was tragically killed in a car crash inside the Pont de l'Alma tunnel in Paris in 1997.
Charles and Camilla went on to marry in 2005, and have since enjoyed what appears to be a wonderful relationship.
The Duchess of Sussex confirmed with fans in January that her due date was late April early May, and Charles and Camilla's change in travel plans seem to suggest that the future grandparents are preparing for Baby Sussex's arrival.
On Monday, the Prince of Wales and the Duchess of Cornwall announced they will visit Germany on May 7.
Since it is to be expected that Charles and Camilla will be nearby for the arrival of the Duke and Duchess of Sussex's baby, we anticipate that Baby Sussex will be born well before their May 7 trip.