It's a controversial storyline that has both shocked and intrigued fans of The Crown, but how true is the upcoming plot, about the illicit affair between Princess Anne and Andrew Parker Bowles?
Andrew was a British army officer when he first met his wife, Camilla in 1965 at a debutante party.
The pair hit it off immediately and dated on and off for eight years.
However, royal sources claim that not everything was as smooth as it looked on the surface.
It's believed that among other flings, Andrew sparked up an affair with Princess Anne in 1970 - when she was 20 and he was 31.
Speaking on the podcast Royals, royal commentator Angela Mollard said: “It was well-known that Andrew Parker Bowles had multiple lovers.”
She added: “This is really funny. Princess Anne, who we always think is the grumpy one with the bun - she had an affair with Andrew Parker Bowles.”
And there's more evidence of the royal romance, in Tina Brown's biography, The Diana Chronicles.
The journalist wrote: “Andrew adored women. And, like his father, he was a royal groupie.
“One of his earlier diversions was a torrid affair with Princess Anne, who, in her stern way, has always enjoyed a roll in the hay.”
However, the affair sparked something of a royal love triangle – as Camilla wanted to get revenge.
And royal biographer, Penny Junor, has previously suggested that Camilla might never have started a relationship with Prince Charles, if her boyfriend wasn't romancing his sibling.
She said: “[Camilla] was passionately in love with [Andrew] but he was a cad, he was bonking other people, some of her friends.
“There was a lovely story when she was walking through London in the evening and spotted his car parked outside the flat of a very good friend of hers. So she let the tyres down and wrote a message in lipstick on his windscreen.
“So when she was introduced to Charles and he thought she was pretty special...he thought she was a bit of alright and she thought, 'Andrew is at the moment off with Princess Anne, you know her brother, teach Andrew a lesson.' So she had a fling with Charles.”
Camilla and Charles first met at a polo match in Windsor Great Park in 1970 and she allegedly told him:
“My great-grandmother was the mistress of your great-great-grandather. I feel we have something in common.”
But things got complicated and Junor recalled: “It was clear that Princess Anne was in love with Andrew Parker Bowles, Camilla was in love with him too, and Charles was in love with Camilla.”
Things came to a head in 1973 when Andrew finally decided to ditch his womanising ways and marry Camilla.
Princess Anne also married Mark Phillips later that year, and Charles and Diana married eight years later in 1981.
However, the royals' love lives have remained intertwined. As Charles continued to hold a torch for Camilla, with the pair finally settling down and getting wed in 2005.
