The moment Diana discovered Prince Charles's love affair with Camilla

Princess Diana, Princess of Wales. Photo / Getty Images

You would have to be a monster not to feel sorry for her. Tucked away behind a throng of people, Lady Diana Spencer, then 19, fought back tears and bit her lip. She was standing at a London Airport wearing a distinctive red coat — watching footage of the moment, her heartbreak is palpable.

It was March 29, 1981 and Diana was at the airport to farewell her fiance, Charles, Prince of Wales as he departed for a five-week international tour to Venezuela and Australia. Engaged for only a month, he briefly touched her arm, chastely kissing her on both cheeks (ooh ahh, stop it you two!) and then cheerfully boarded his plane in a natty suit.

Back on the tarmac, Diana was tearing up.

At the time, the press interpreted the Princess-to-be's mournful performance as that of a woman whose beloved was being cruelly ripped away from her to fulfil his noble duty of visiting far flung corners of the Earth to unveil plaques.

Instead, Diana was, at the moment the cameras caught her airport meltdown, both heartbroken and enraged by her fiance.

The truth was revealed by Diana herself. In 1991, the Princess started making secret recordings that a friend would pass on to journalist Andrew Morton. (They would form the basis of his bombshell book, Diana: Her True Story.) Decades later, the tapes were played as part of a National Geographic documentary called Diana: In Her Own Words.

"You may recall seeing a picture of me sobbing in a red coat when he went off on his aeroplane," the then-30-year-old says on the tapes. "That had nothing to do with him going. The most awful thing had happened before he went."

The "most awful thing" that Diana was referring to involved the woman that Diana would come to hate with a vengeance — Camilla Parker Bowles.

"I was in (Charles's) study talking to him about his trip," Diana explains.

"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."

The image is poignant: the doe-eyed teenager, so sure she had snaffled her Prince and was getting him to the altar, bravely walking away. Meanwhile, the man she wanted so badly to shower her with the love she had been denied as a child, chatted to the woman he had fallen for.

The next day, when Diana accompanied Charles to the airport like a dutiful fiance, she couldn't keep what she was feeling bottled up anymore.

Interestingly royal biographer Tina Brown argues that Diana was experiencing another emotion at that moment that would ripple through the Wales's marriage: Anger.

"Diana's tears at the airport were not of grief but of rage," Brown writes in The Diana Chronicles.

Meanwhile, with Charles off waving to his subjects in the Commonwealth during March 1981, what now seems to be one of the most improbable tete-a-tetes in history took place, with Diana sitting down with Camilla for a, superficially, fun girls' lunch. A decade later, Diana would tell Morton of that meal: She (Camilla) said: "You are not going to hunt are you?" I (Diana) said: "On what?" She said: "Horse. You are not going to hunt when you go and live at Highgrove are you?" I said: "No." She said: "I just wanted to know," and I thought as far as she was concerned that was her communication route."

Translation, as far as Diana was concerned, this was Camilla working out when and where she could see the Prince. Given Camilla and Charles were both avid hunters, this could provide them with the perfect cover to spend time together.

(It is worth pointing out that there is some debate over whether Camilla and Charles were actually sleeping together at that point. Charles himself later claimed that he only rekindled his relationship with Mrs Parker Bowles, as he publicly called her, in 1986.)

The phone call and the lunch weren't the only pre-wedding clues about Charles's affection for Camilla.

Later, finding a gift on the desk of Michael Colborne, Prince Charles's personal secretary that was clearly not meant for her. Inside was a gold bracelet with a blue ceramic disk with "GF" on it. There was only one woman the 31-year-old groom-to-be called his "Girl Friday" — Camilla.

(Brown has argued that this was not a romantic gesture and that the Prince had decided to give a number of friends gifts before he took what he called "le grande plonge".)

As the wedding drew closer, Diana turned to her sisters for support, who instead gave her the cold hard truth. "Well, bad luck, 'Duch.' Your face is on the tea towels so you're too late to chicken out."

And she didn't. On the 29th of July, 1981 she gamely climbed the stairs of St Paul's Cathedral wearing a charming monstrosity of a dress and the Spencer family tiara to pledge her troth to a man who had already dashed so many of her idealistic hopes for their future together.

You can say many things about Diana, but she certainly didn't chicken out, not even with what was to come.