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.
People magazine has named Lupita Nyong'o as the "World's Most Beautiful" for 2014. Read More »
Justin Bieber apologised to those he offended by visiting Japan's Yasukuni war shrine. Read More »
Kate and William drop some sick beats!
12 pics of Prince William and Duchess Kate!
New great pics of baby Prince George!
20 new pics of Kate and William.
6 best pics from the royal tour!
Check out these pics of Prince George!
10 pics from Will and Kate's visit.
Kate and Will wows the crowd!
South AfricaDiscovery ITMarket Related
South AfricaCity PressMarket Related
South AfricaCity PressR572517 - R935763
HousesR 1 800 000
HousesR 8 900 000
HousesR 1 850 000