Prince of Wales and the late Princess Diana reportedly had a "very heated discussion" for hours after she found a bracelet that didn't belong to her.
The 68-year-old royal was married to Diana - who tragically died in 1997 aged 36 - for 15 years before they divorced in 1996, and it is believed the golden-haired beauty sparked up a blazing row with Prince Charles after she found a piece of jewellery on a desk that was not hers, and suspected her beau was cheating on her.
Speaking about their relationship in a new biography about Charles' current spouse Duchess Camilla titled 'The Duchess: The Untold Story', which has been penned by Penny Junor and has been serialised by the Daily Mail, Junor wrote: "What enraged Diana most of all was a bracelet destined for Camilla that she found on Michael Colborne's desk one Friday afternoon.
"Diana confessed she'd had a look at what was on his desk but said nothing further about it.
"They had a very heated discussion."
However, Charles reassured Diana there was "no other woman" in his life aside from her.
The text continued: "He told her unequivocally that there was no other woman in his life and he expected her to believe him."
But Diana felt she wasn't met with a "clear answer".
Penny explained: "Worse, she'd asked Charles if he was in love with Camilla and he hadn't given her a clear answer."
And it is believed that Charles - who has sons Prince William, 35, and Prince Harry, 32, with Diana - underestimated how "insecure" Diana would feel about her partner giving his ex-lover a present.
Penny explained: "But he didn't try to imagine how an insecure 19-year-old might feel about him giving a bracelet to the one old girlfriend she was fretting about."