The Crown's season three will show how Camilla Parker Bowles became ‘Britain's most hated woman'.

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When Princess Diana and Prince Charles posed on the steps of St Paul’s Cathedral on their wedding day, their relationship looked every bit the royal fairytale romance.

The 15-year marriage they were about to begin, however, was anything but.

Even before the pair wed in 1981, there was a third person in Charles and Diana’s relationship – Camilla Parker Bowles.

Prince Charles and Camilla Shand first met at a polo match in 1970 after being introduced to each other by a mutual friend.

“He was very taken with her and after that first meeting he began ringing her up,” royal biographer Penny Junor wrote in her book, The Duchess: The Untold Story.

But despite building a strong connection quickly, royal protocol at the time created a road block in the couple’s relationship.

Without an aristocratic bloodline or her virginity, Camilla simply didn’t have the ‘prerequisites’ required for marrying into the royal family.

Shortly after meeting, Prince Charles joined the Royal Navy in 1971, putting more strain on the new relationship.

Two years later, while Charles was serving in the Caribbean, Camilla reconnected with British Army officer Andrew Parker Bowles, who she went on to marry in 1973. Together they had two children – Tom born in 1974 (who is a godson of Prince Charles) and Laura born in 1978.

Although separated, Camilla and Charles remained close friends.

In 1980, Charles began dating Lady Diana Spencer. In February 1981, Charles proposed and the couple wed on July 29, 1981.

But just weeks before Diana married Charles, she discovered something incriminating.

According to The Mirror, Diana discovered a parcel containing a bracelet Charles had 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," Diana 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."

According to Andrew Morton, the bracelet was engraved with the letters 'G' and 'F', for Charles and Camilla's nicknames Gladys and Fred.

Despite making the devastating discovery, Diana went through with the marriage and the couple went on to have two children together – Prince William in 1982 and Prince Harry in 1984.

But while their marriage appeared happy from the outside, by 1986 Camilla and Charles had reportedly began seeing each other again, leading Diana to confront Camilla.

"I was terrified of her. I said, 'I know what’s going on between you and Charles and I just want you to know that,'" Diana told her biographer Andrew Morton.

"She said to me: 'You’ve got everything you ever wanted. You’ve got all the men in the world fall in love with you and you’ve got two beautiful children, what more do you want?' So I said, 'I want my husband'. And I said, 'I’m sorry I’m in the way … and it must be hell for both of you. But I do know what’s going on. Don’t treat me like an idiot.'"

Several years later in 1992, Camilla and Charles' affair was exposed when tapes of conversations between the two were leaked.

Diana and Charles announced their separation shortly after the leak before formally divorcing in August 1996. Just one year later, Diana was killed in a car crash in Paris.

Two years after Diana's death, Charles and Camilla emerged in public for the first time together in 1999 before Camilla moved into Clarence House several years later in 2003.

In 2005, the couple married and Camilla was given the new title of 'Duchess of Cornwall'.

It hasn't been an easy road for the Duchess of Cornwall, however. For years, Camilla was described in the press worldwide as 'the most hated woman in Britain'.

"It was horrid. It was a deeply unpleasant time and I wouldn't want to put my worst enemy through it," Camilla confessed last year.

And now, as The Crown's next season delves into this part of the royal family's history, it's bound to open some old wounds.

British actors 28-year-old Josh O'Connor and 33-year-old Emerald Fennell have been selected to play the young prince and his mistress. It will air in 2019.