According to Penny Junor, who has written about several members of the Royal Family, Camilla wanted to teach a lesson to Andrew Parker-Bowles.

London - The Duchess of Cornwall might never have got together with Prince Charles had her first husband not cheated on her with Princess Anne, a leading royal biographer has suggested.

According to Penny Junor, who has written about several members of the Royal Family, Camilla wanted to teach a lesson to Andrew Parker-Bowles – who was her boyfriend at the time.

And Miss Junor went on to claim that as a form of revenge – or "tit-for-tat" as she describes it in her latest book – Camilla enjoyed her first "fling" with Prince Charles in the early 1970s.

Miss Junor recalled the dalliance during a talk at the Henley Literary Festival, explaining it came about because Andrew Parker-Bowles was having numerous affairs and Camilla’s friend – who also knew Charles – intervened. 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."

During her talk at the festival, Miss Junor – whose book was approved by Camilla – said her research had led her to discover "just how the upper classes bonk".

Miss Junor explained: "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.

"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."

In an extract from her recently-published book, The Duchess: The Untold Story, she wrote: "There was certainly an element of tit-for-tat in Camilla’s fling with Charles.

"Indeed, her principal motivation was to have some excitement and make Andrew jealous.

"She knew the affair with Charles would never go anywhere, could never go anywhere."

Miss Junor said she hopes to dispel myths about the duchess with the book and demonstrate the positive impact she has had on Charles.