A former bodyguard to Diana, Princess of Wales, says she discovered Charles hidden away with Camilla Parker Bowles at a society party.
Ken Wharfe makes his claim The Sunday Times newspaper.
He says it was the moment she realised her marriage was over.
Diana was under such pressure over the troubled marriage that she once jumped 20ft from a hotel balcony, he said.
During her affair with James Hewitt, Mr Wharfe persuaded the princess to use code names on the phone, he also revealed.
She became "Julia" while the Prince's residence at Highgrove was "Low Wood".
Mr Wharfe says the pair had to notify royal protection officers when they wanted to meet, but that Hewitt was happy to do so.
He stated: "He fully accepted that the safe houses in which he met his lover had all to be checked for security before an assignation could take place,"
He adds that he was dismayed the good qualities of Diana were being airbrushed from history in order to prepare the public to accept Camilla as Charles' consort.
Mr Wharfe said: "Since her death, friends or so-called friends of the Royal Family were giving statements about Diana saying she wasn't right, she had a problem, that men in white coats were coming. This was a different woman."