The X Factor boss had co-operated with and given hours of interviews to journalist Tom Bower, whose Sweet Revenge: The Intimate Life Of Simon Cowell is currently being serialised in The Sun.
Cowell has said on Twitter: "This book is not written by me. It is unauthorised. The writer is Tom Bower."
He had previously written: "Just seen the TV ad for the unauthorised book on me. Yikes!"
The book quotes Cowell as admitting a fling with his former colleague Dannii Minogue, a claim later confirmed by the mogul's publicist Max Clifford.
Minogue was said to be "distraught" by the reports and Clifford later claimed that Cowell and his team "scored a massive own goal" by talking to Bower.
The book also suggests that Cowell was besotted with Cheryl Cole, saying: "I felt I was the mouse with a beautiful cat."
It has been claimed that the Daily Mail lost out on the chance to serialise the biography to The Sun because of the "arrogance" of its low offer.
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Watch our 2010 interview with Dannii Minogue below:
Our gallery of Simon Cowell's famous previous relationships: