Simon Cowell is to leave American Idol to launch The X Factor in the US.

He said The X Factor, the show he created in Britain, will join TV station Fox's schedule next year and he will be on it.

Cowell's decision is the biggest threat yet to America's most popular TV programme. This season, original host Paula Abdul was replaced by Ellen DeGeneres.

But Cowell, with his caustic commentary, has long been seen as the big star.

He said it would have been difficult for him to do both shows.

While he makes a reported 36 million US dollars a year on American Idol, he owns The X Factor and could make much more if the show takes off.

Cowell and top Fox executives made the announcement in Pasadena, California, at a meeting of the Television Critics Association, saying they had reached an agreement only a few hours before.

Cowell said: "I was offered a lot of money to stay on. But that wasn't the reason behind it. I wanted to do something different. I wanted a new challenge."

Peter Price, chairman of Fox, would not speculate on possible replacements for Cowell.

"We have to take our time on that," he said. "We have to make sure the chemistry of the judges is as good as it can be."

Cowell said he did not want to leave American Idol at a time when it was fading in the ratings.

"You want to leave on a high," he said. "I'm very proud of what the show has achieved."