Simon Cowell and Cheryl Cole quit The X Factor?

Simon Cowell and Cheryl Cole have reportedly quit the UK 'X Factor'.

The pair - who are joined on the ITV1 show's judging panel by Louis Walsh and Dannii Minogue - have decided to leave their roles to concentrate on the launch of the US version of the talent competition, leaving 'X Factor' bosses terrified it will spell trouble for the new series, with auditions starting in just two weeks.

An insider explained: "The success of the American show means everything to Simon. He is financially and emotionally invested in it and is going up against his biggest rival, Simon Fuller, who runs 'American Idol'.

"He seriously considered commuting between the States and London. Len Goodman and Bruno Tonioli do 'Strictly Come Dancing' and its American equivalent at the same time. But all they have to do is watch and judge dancers.

"Simon would have to produce two big shows at the same time and he and Cheryl would have to mentor and judge acts in both countries. It would be just too much for them and it wouldn't work."

Producers are now said to be considering an entire "shake-up" of the judging panel with Louis Walsh facing being axed from the show altogether, with Pussycat Doll Nicole Scherzinger, Take That's Robbie Williams and singer Adele all believed to be in the running for a role on the UK show.

The source added to the News of the World newspaper: "There will be a massive shake-up. Simon is a huge, huge loss. He gives the judging panel such authority and there are big fears that many will switch off without him. And two million extra viewers tuned in when Cheryl joined the panel. To lose her as well is a nightmare.

"Dannii looks the most likely to survive. Louis' position is in danger - but no one has been booted off yet. With Simon and Cheryl not there the whole thing could feel very flat."

The first round of 'X Factor' auditions begins on March 6 in front of a panel of show producers at London's O2 Arena.