It's no secret that King Charles's romantic history has garnered immense interest over the years.

The then Prince of Wales married his first wife, Princess Diana, when he was 32 years old, a union that would reveal irreparable compatibility issues and emotional strain on both sides.

As it was revealed during the latter stages of Charles's marriage, the King had just one true love - Camilla Parker Bowles.

According to a new book, Divide & Rule, by royal author Catherine Mayer, Camilla was always a "non-negotiable" part of his life, with the King having "no plan B" if she couldn't be integrated into royal life.

She said that several friends of Charles deemed Camilla's role essential, with "painstaking" efforts made to rehabilitate her public image after her extramarital affair with the then Prince was revealed.

Speaking to Fox News Digital, Catherine said the Queen became "non-negotiable" after Charles's divorce from Diana in 1996, just one year before she tragically died in a car accident in Paris.

She said Charles understood there was only "one woman who could really sustain him", and that he required a level of support that only she could give.

Catherine described the King and Queen as having one of the "most unlikely love stories" imaginable. For years, many people believed in the fairytale marriage between Diana and Charles. 

The two married on 29 July 1981. Diana was just 20 years old at the time. The couple would manage just over a decade before the cracks in their marriage became inescapable, and they officially announced their separation in 1992.

It was at this stage that those avid royal fans were forced to accept "the truth of what happened" between Charles and Diana. When news of Charles' extramarital relations with Camilla became public in 1993, questions about his suitability as heir were raised, and his mistress found herself at the centre of immense public outrage.

As Catherine put it, Camilla came to be seen as "the other woman who destroyed that relationship." The author believes that Charles and Diana could never have managed a happy partnership, but said the former Princess of Wales was "treated very badly".

Charles married Camilla at a private civil ceremony on 9 April 2005. Catherine described Camilla as having a "magical effect" on the King. "It's transformative."

According to the author, Camilla's hold over Charles was so powerful that any time she would walk into a room, he would "light up". The King and Queen first met through a mutual friend in 1970. 

Many say that it was love at first sight for the couple, but it would take more than three decades for them to live their relationship publicly.