Simon Cowell was stunned to discover Britney Spears was "shy" on the set of The X Factor as he feared she was still living a wild lifestyle.

The music mogul has grown close to the Toxic hitmaker after sitting alongside her on the show, and admits he found her charismatic.

He tells People.com, "It's quite interesting because when someone has done what she's done as she has with her career, and being such a public person, you kind of think she is going to be very confident, but in real life she's shy. There's kind of a mystique about her which I think is what kind of attracted me to her in the (first) place."