Princess Anne and Andrew Parker-Bowles had a heated love affair while he was dating Camilla Parker-Bowles, royal writer Penny Juror claims.
Throughout the years, the former British Army Officer's extramarital affairs become somewhat notorious, after he married Camilla Shand - now Duchess of Cornwall - in 1973.
Royal writer Junor believes that Camilla may never have struck up a romance with Prince Charles if her then-boyfriend hadn't struck up an affair with his royal sibling.
Junor, who regarded as a reliable source, claimed that Camilla planned to teach her 'cheating boyfriend a lesson.'
She recalled the love affairs 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.
'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.'
Fellow royal writer Phil Dampier believes Junor's story may be true, and knowns 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.'
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's claims are yet to be confirmed.
Princess Anne went on to marry Captain Mark Phillips, and the couple had two children - Peter and Zara. However, the pair divorced 19 years later.
The royal has now been happily married to Timothy Laurence since 1992, eight months after her divorce from Phillips.