Princess Anne with old lover Andrew Parker Bowles look on in the Parade Ring as she attends Ladies Day on day 3 of Royal Ascot at Ascot Racecourse on June 18, 2015 Getty

A royal affair has allegedly come to light four decades late after royal biographer Penny Junor claimed Andrew Parker-Bowles cheated on then-girlfriend Camilla with Princess Anne.

Former British Army officer Parker-Bowles was renowned for his extra-marital affairs after marrying Camilla Shand – now Duchess of Cornwall – in 1973. But the extent of his relationship with Princess Anne in the early 1970s isn't widely known, which took place while he was courting the future Queen of England.

Royal writer Junor believes that Camilla may never have struck up a romance with Prince Charles if her boyfriend at the time hadn't been dating the royal's sibling.

Junor is a reliable royal source and has written about several members of the Royal Family, and claims that Camilla planned to teach her cheating boyfriend Parker-Bowles a lesson.

Camilla's form of revenge was "tit-for-tat" as she embarked on a fling with Charles, not knowing it would shape the monarchy with its modern approach in years to come.

Junor recalled the dalliance during a talk at the Henley Literary Festival, stating: "[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.

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Andrew Parker Bowles OBE with ex-wife Camilla, Duchess of Cornwall attend Ladies Day, day 2 of The Cheltenham Festival at Cheltenham Racecourse on March 12, 2014 in Cheltenham, England Getty

"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."

Addressing this claim, royal writer Phil Dampier thinks it could be true and knows that Parker-Bowles and Princess Anne are very close friends.

He told IBTimes UK: "I have read this before and it wouldn't surprise me. Anne and Andrew Parker Bowles were very close – and they still are!

"You see them together at Royal Ascot every year and they are best friends. He was her first love and their bond goes very deep. It was a weird royal set-up which Diana went into not realising the depth of the relationships."

camilla parker bowles
Prince Charles, Prince of Wales and Camilla, Duchess of Cornwall, photographed by Mario Testino in the Morning Room at Clarence House Getty

Royal commentator Richard Fitzwilliams added: "Whether or not this is all true, the chemistry between Charles and Camilla was always ideal so they might well have had an affair anyway.

"Musical beds is one of the sports of the upper classes. The tragedy was that in those days Charles could not marry anyone with 'a past' and the consequences of this when he chose Diana, as we all know, were absolutely catastrophic."

Junor even revealed who introduced the now married-couple of 12 years at the talk.

She continued: "Charles had this friend, Lucia Santa Cruz, her father was an ambassador, she was at Cambridge, they became very good friends. [They were] not lovers, she has been credited with taking his virginity ... not true.

Camilla and Charles
Camilla, Duchess of Cornwall and Prince Charles, Prince of Wales listen to traditional throat singers during an official welcome ceremony at Nunavut Legislative Assembley during a 3 day official visit to Canada on June 29, 2017 Getty

"She ended up living in a flat above Camilla in Pimlico, she became very good friends, shared clothes, [spent] hours chatting with each other, shared secrets, Lucia knew exactly what was going on with Andrew and could not bear to see her friend treated this way so she had this wonderful idea to introduce Camilla to Charles.

"I think she would have done it even if Andrew had not been seeing Princess Anne. If it had not been Anne, it would have been someone else," she added.