The 60-year-old TV personality was a judge on 'X Factor' in 2007 alongside the media mogul, the Australian beauty and Louis Walsh, however her relationship with Dannii deteriorated to a point where she dreaded going to work, when the 41-year-old singer was having a fling with Simon.
In her autobiography 'Unbreakable' Sharon wrote: ''The more it progressed, the worse it became between her and me. It was unbelievably bad.
''Dannii had taken to walking past me in the hallway without even making eye contact. I'd like to say it didn't bother me, but it did.
''A lot. I had really enjoyed the previous three series, but this one was rapidly turning into an odious chore. I found it hard dealing with a sulker.''
Sharon alleges that Dannii's romantic attachment to Simon gave her a power boost where she became demanding and acted inappropriately.
She explained: ''Outwardly, Dannii seemed all 'Ooh, I love kids and puppies.' But in my opinion she was dark, very dark. What you saw was definitely not what you got.
''And so the show went on, and on, and bloody on, for what seemed an age. The closer she got to Simon, the more I felt she started telling the producers what to do. Suddenly, from being the new girl, she was saying, 'This isn't right,' and 'Simon doesn't like it this way.'
''Meanwhile the antics behind the screen had gone up a gear, with her sticking her tongue in Simon's ear and giggling like a bloody teenager as Louis Walsh and me stood there like a couple of gooseberries.''
Sharon approached Simon to make him choose between them however he refused to do so leaving her ''in no doubt this wasn't a matter for debate.''
Meanwhile Simon is expecting his first child with New York socialite Lauren Silverman.