The TV mogul, 52, confessed they were lovers when they worked on the show. He admitted: “I had a crush. It was genuine love.”
And he reveals he lusted after Cheryl Cole and wanted to bed her too, when his feelings for Dannii cooled.
The astonishing revelations are made in a book published next month called Sweet Revenge — The Intimate Life of Simon Cowell, by acclaimed biographer Tom Bower.
The blockbuster, exclusively serialised in The Sun this week, also tells how Cowell planned to seduce Dannii’s friend, singer Natalie Imbruglia, just last year after inviting her on to his yacht.
It reveals how Cowell “fell in love” with Dannii, now 40 and a mum of one.
This month Dannii — on The X Factor from 2007 to 2010 — split from Kris Smith, 33, the father of her 21-month-old son Ethan, after four years.
Then Cowell became obsessed with Cheryl, 28, when she joined the show. He was “mesmerised” by her many visits to his dressing room and moved her to sit next to him on the panel.