Why Prince Philip encouraged Prince Charles' affair with Camilla Parker-Bowles.

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The world watched starry-eyed the day Princess Diana and Prince Charles exchanged vows in 1981 – transfixed on a royal romance which captured every element of a fairytale definition.

Or so we thought.

Over a decade from the day they said "I do", their marriage fell apart in one of the biggest modern-day scandals surrounding the royal family.

This was largely attributed by the public to a third person involved in their relationship – Camilla Parker-Bowles, who Prince Charles went on to wed in 2005, following Princess Diana's tragic death in 1997.

But while it has previously been reported that Diana was aware of Prince Charles' infidelity years before their divorce, and even had affairs of her own - a letter sent by Charles' father, Prince Philip, suggests that Philip also knew about the affair, and encouraged his son to cheat.

Netflix series The Royal House of Windsor revealed that Charles married Diana amid family pressure, after he had sought advice regarding his conflicting feelings between Diana and Camilla.

According to Express UK, Christopher Wilson, royal biographer and journalist, said: "Charles was a desperate man, there was huge downward pressure on him from Prince Philip, from the Queen and the whole court to sort out his life, to stop running around and try and find someone."

At the time, Charles’ on-and-off love interest Camilla was already married, so the family had reportedly urged him to move on and meet someone new.

After he met Diana at a party at Petworth House and their romance began to blossom, Prince Philip reportedly sent him a letter instructing him to marry Diana, or leave her.

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Royal historian Sarah Gristwood said: "Prince Philip effectively urged his son to get on with it - to either propose to Diana or back off."

It is believed he offered this advice while aware that Prince Charles still had feelings for Camilla, a view Princess Diana herself held after their divorce.

In the documentary Diana: In Her Own Words, a conversation between her and her voice coach uncovered her feelings towards Prince Philip's role in the affair.

“My father-in-law said to my husband, ‘If your marriage doesn’t work out, you can always go back to her [Camilla] after five years,’” Princess Diana said.

“Which is exactly, I mean for real, I knew that it happened after five. I knew something was happening before that, but the fifth year I had confirmation,” she added.

According to the book Charles at Seventy: Thoughts, Hopes and Dreams, Prince Charles was "desperate" to back out of his wedding to Diana, on discovering "just how awful the prospects were" considering how little the pair knew each other.

In Diana: In Her Own Words, it seemed Diana, too, was having second thoughts about their relationship, telling her sisters “I can’t do this” before the ceremony.

According to Robert Jobson’s Royal Podcast, throughout their ill-fated 11 year marriage, Charles would make secret phone calls to his former partner Camilla while travelling with his wife and their two children, Prince William and Prince Harry.

But despite seemingly making the phone calls in private, former royal security guard Ken Wharfe said Diana was well aware of their affair.

“It was sad in a sense for Diana of course because Camilla was always on the back of her mind,” Wharfe said on Robert Jobson’s Royal Podcast.

“At one point during this trip, Diana was aware that Charles was on the telephone to Camilla,” he added.

The family holiday wasn’t the first time Diana learned about the secret phone calls between her husband and Camilla.

Instead, Wharfe said that it was just something she “lived with”.