The 40-year-old singer-turned-fashion designer enjoyed several nights of passion with The X Factor boss after joining the judging panel in 2007, according to a new unauthorised biography, Sweet Revenge: The Intimate Life of Simon Cowell.
But having just split with Kris Smith, the father of her 21-month-old son Ethan, the revelation is said to have pushed an already upset Dannii to breaking point.
A source said: "She's understandably distraught by all of this coming out just as she's struggling to cope with the breakdown of her relationship with Kris."
"It was all such a long time ago and to have to deal with it now in the wake of the last few weeks is just too much. She is incredibly raw and this is the last thing she wants to endure."
In the book, Simon, 52, is quoted as saying: "I had a crush on her. It was Dannii's hair, the sexy clothes and the tits. I was like a schoolboy. She was foxy. She was a real man's girl. Very feminine."
According to the Daily Mirror newspaper, Simon allowed the author Tom Bower hundreds of hours of access to his life, although the music mogul apparently claims to have had no control over what was included in the finished product.
He has so far kept quiet over the situation, only breaking his silence to tweet: "Just seen the TV ad for the unauthorised book on me. Yikes!"
24.com publishes all comments posted on articles provided that they adhere to our Comments Policy. Should you wish to report a comment for editorial review, please do so by clicking the 'Report Comment' button to the right of each comment.
Family and friends, including filmmaker Tyler Perry, are expected to pay tribute to Bobbi Kristina Brown, the only child of Whitney Houston and R&B singer Bobby Brown in a private service at St James United Methodist Church in Alpharetta. Read More »
Dionne Warwick is threatening to boycott Bobbi Kristina's funeral over the ''sweet 16'' theme which is being enforced by her aunt Pat Houston. Read More »
All the facts.
South AfricaVitality RSA Systems
HousesR 2 480 000
TownhousesR 1 280 000
TownhousesR 1 750 000