In an interview released as part of the documentary special Diana: In Her Own Words, the late Princess recounted the agonising moment her fears were confirmed.
After weeks of missed hints – like her soon-to-be husband spending time at the Parker-Bowles country home “quite a lot” and Camilla’s seemingly intimate knowledge of their plans – Diana learned the truth when she happened upon a bracelet Charles had designed for his mistress.
“I was still too immature to understand all the messages coming my way. And then someone in his office told me that my husband has had a bracelet made for her,” she explained to her close friend, who recorded the interview on behalf of reporter Andrew Morton.
“I walked into this man’s office one day and I said, ‘Ooh, what’s in that parcel?’ And he said ‘Oh, you shouldn’t look at that’.
“So I opened it and there was the bracelet. I was devastated, and I said, ‘Well, he’s going to give it to her tonight’.
Diana said she confronted Charles over the bracelet, asking him, “Why can’t you be honest with me?”
She said from his response, it was clear he didn’t care.
"But no, absolutely cut me dead. It was as if he’d made his decision and if it wasn’t going to work it wasn’t going to work."
Diana said at that moment she seriously considered breaking off her engagement, but her sisters talked her out of it.
"I went upstairs, had lunch with my sisters who were there. And I said ‘I can’t marry him, I can’t do this, this is absolutely unbelievable."
"And they were wonderful and said, "Well, bad luck, your face is on the tea towel, so you are too late to chicken out'."
Diana shared another troubling moment three months before their wedding, right before Charles took off for a five-week trip. She explained that photos of her, then aged 19, crying at the airport in a red coat had "nothing to do with him going".
"The most awful thing had happened before he went," she recalled.
"I was in his study talking to him about his trip. The telephone rang. It was Camilla. Just before he was going for five weeks.
"So I thought, 'Shall I be nice or shall I just sit here?' So I thought I’d be nice, so I left them to it and it just broke my heart."
But of course, the then Lady Diana Spencer did decide to follow through with her wedding, and married the Prince of Wales at St Paul's Cathedral on 29 July 1981.
By 1996, after welcoming sons Prince William and Prince Harry, the couple's divorce was finalised. Almost a year later to the day, on 31 August 1997, Diana was killed in a car accident in Paris.
The royal family will be privately grieving their loss on the 21st anniversary of her death today.
You can watch 'Diana: In Her Own Words' on Netflix.