Queen Camilla was left stunned by Princess Diana's furious outburst, according to the late royal's former protection officer.
The late Princess of Wales gave the now-Queen a warning after she caught her and King Charles sneaking off from a party together.
Diana married Charles on July 29, 1981, at St Paul's Cathedral and they had two children together, Prince William and Prince Harry.
Ken Wharfe, who worked for the late princess for six years, spoke on Lorraine about the moment Diana confronted Camilla and her then-husband.
Charles and Diana married in 1981
Princess Diana and Charles had attended a party being thrown by Camilla's sister, with the current Queen also attending.
According to the protection officer, people were surprised that Diana went to the party, as many people there knew about Camilla and Charles's extramarital affair.
Wharfe said: "Diana didn't have any particular friendships at that party but when we arrived there, it was almost like freeze-framing a scene in a movie because there was this surprise that Diana had even arrived.
Queen Camilla and Princess Diana photographed together in 1980
"But then soon after within the hour, I heard my name being called, I was in the kitchen somewhere.
"And I went out and there was Diana, who said 'You've got to come with me, I can't find my husband or Camilla'.
"Now, I couldn't say no, clearly this woman was in some distress, and eventually we found the prince and Camilla sat on a sofa in the basement of this property just talking."
Wharfe claimed that this was when Diana issued her warning to her then-husband.
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He added: "I didn't know quite what Diana was going to do at that point but with a great deal of confidence, Diana just went up to both of them and said to Camilla 'Please don't treat me like an idiot, I know what's going on'.
"And Camilla sort of said something, to which still to this day I have never really understood what she meant by that is 'Well you know, you have two wonderful boys'.
"Well, it was an incredible moment for me and certainly for them as well.
"That was a defining moment in their life because I think at that point... this was an indicator the end was nigh."