Writing in 1992, five years before Diana’s death, author Andrew Morton noted in his book Diana: Her True Story that the rumours erupted during a special yachting trip.
At the time Diana and Charles were “sleeping in separate cabins” with the Princess opting to spend more time with her children, Princes William and Harry.
The holiday was a family affair, and included Princess Alexandra, her husband Sir Angus Ogilvy, the Romseys and their children, Mr Morton noted.
But one encounter with Camilla that left Diana raging, leaving her “suspicions of Charles' affair” clear.
Mr Morton wrote: "Diana had little day-to-day contact with her husband, sleeping in a separate cabin and preferring to eat with her children rather than the adults.
"The underlying tension was not helped when Diana picked up a ship-to-shore telephone to discover her husband in the midst of a long call to his long-time friend, Camilla Parker Bowles.
"It provided further fury for Diana's suspicions regarding her husband's relationship with Camilla, misgivings which had been consistently derided as the fantasies of a sick woman."
He added: “Princess Margaret and a group of friends who joined the Wales’ party for a time noticed the gloomy mood on the Latsis yacht.
“‘The marriage is all over bar the statement,’ said one of the yacht's guests after the royal party returned to London.
“‘The holiday was a shambles for the adults.
“‘It was organised for the children a long time ago so on that level it was a success’.”
Camilla's call to Charles was just another nail in the coffin of his marriage to Diana.
The Prince and Princess of Wales split, and were divorced in 1996, just one year before Diana's untimely death in a Paris car crash.
Charles went on to wed Camilla, who had divorced her first husband, Andrew Parker Bowles, in 1995.