Britney Spears almost quit 'The X Factor' on her first day of filming, and had to be talked around by Simon Cowell.

Britney Spears almost quit 'The X Factor' on her first day.

The 'Toxic' singer signed a rumoured $15 million contract to appear as a judge on the star search show, but had a panic attack on her first day and walked off set.

After admitting his team "half lied" when they said she only needed to go to the bathroom during the live auditions, show boss Simon Cowell told website TMZ: "No one is going to admit that on the first day your highest paid star after 20 minutes has walked off set.

"[She] had a moment on the first day where she said to me 'I'm not sure I can do this.'"

Simon managed to talk her around and filming for the US version of the show then went ahead as planned. However, Simon also warned Britney was much harder to please when it came to judging singers than he thought she would be.

He said in a press conference: "You know how when you buy a dog, you expect it to lick you, and then it turns out to bite you? It was rather like that.

"It was a real surprise. She's really, really difficult to please. But in a way I think it made it more interesting because she just wouldn't say 'Yes' for people to like her. And when you do these auditions in front of 5,000 people, they boo you."

'The X Factor' - which also features Demi Lovato and LA Reid on its judging panel - will return to US screens on September 12.