Penny Junor said that Camilla Shand, as the Duchess was known in 1971, had grown tired of the dalliances behind her back of her future husband Andrew Parker Bowles.
Junor told the Henley Literary Festival: "So when she was introduced to Charles he thought she was pretty special and he thought she was a bit of all right,"
"And she thought 'Andrew is at the moment off with Princess Anne, I'll teach him a lesson' so she had a fling with Charles," the Times reported.
Junor said Camilla had been "passionately" in love with Parker Bowles and generally put up with his behaviour even after the couple married in 1973.
"He was a cad, he was bonking other people, some of her friends," Junor said.
She said Prince Charles had affairs in his 20s and 30s, often with married women one of whom was identified as Dale, or Lady Tryon.
Junor wrote in The Duchess: The Untold Story that Charles was "heartbroken" when she decided to marry Parker Bowles and they carried on seeing each other, until his engagement to Diana.
The duchess would avoid times where the prince was present and dismissed claims by Princess Diana that Camilla had slept with the prince on the eve of his wedding.
Junor reportedly interviewed close friends of the Duchess for the book. She has also written biographies of Prince Charles and Princess Diana.