PASADENA, Calif. (AP) — Simon Cowell, the acerbic Brit who has helped give “American Idol” some of its sharpest — and nastiest — moments, will leave the hit TV singing contest after this season.

The cantankerous judge said Monday that “The X Factor,” a talent show he created and which is popular in Britain, will join Fox’s schedule next year. Cowell will be a judge on “The X Factor” and its executive producer.

Cowell’s decision is the biggest threat yet to what has consistently been the country’s top-rated TV program and a true cultural force. This season, original host Paula Abdul has been replaced by Ellen DeGeneres.

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

He said it would have been difficult for him to do both shows. While he makes a reported $36 million a year to be 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 to reporters in Pasadena at a meeting of the Television Critics Association; they said they had reached an agreement only a few hours before.

“I was offered a lot of money to stay on,” Cowell said. “But that wasn’t the reason behind it. I wanted to do something different. I wanted a new challenge.”

But an even greater challenge is posed for Idol producers. Without the show’s biggest attraction and most critical judge, will “American Idol” lose steam and plunge even more in ratings? At least one analyst, Shari Anne Brill of Carat USA, said the “Idol” audience probably will decline next season.

“I think it’s that brand of sarcasm combined with professional know-how that makes Simon the audience magnet that he is,” she said. “I really believe the show revolves around him. He’s the linchpin of the show’s success. He has tremendous influence on how the audience votes. He’s interesting to listen to. He’s brutally honest.”

That said, even without Simon, she predicts the show “will still be a formidable player on Fox’s schedule.”

Kevin Reilly, Fox entertainment president, said Cowell’s departure from “Idol” isn’t necessarily a win for the network’s competitors.

“I think it would be premature to be popping corks. Maybe they can say this gives us a little bit of wiggle room, but that’s a big maybe. On the other hand, we’re not losing Simon Cowell, we’re potentially gaining another big headache for them in the fall,” Reilly said.

Peter Rice, chairman of entertainment for Fox, added a dramatic touch to the news conference by asking Cowell to sign his new contract. Cowell’s deal with “American Idol,” which returns for its ninth year Tuesday, will end with the season.

Cowell said launching a show that doesn’t put an age limit on contestants — and allows groups along with individual singers — makes it very different from “American Idol.” The top age for “Idol” singers is 28.

Susan Boyle, 48, who was discovered on “Britain’s Got Talent,” is an example of why age should be irrelevant, said Cowell, a judge on the British show he created and executive producer of “America’s Got Talent” on NBC. Boyle became an unlikely sensation and released one of the year’s top-selling CDs.

Rice wouldn’t speculate on possible replacements for Cowell on “Idol.”

“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 there are many who want the job. But while everybody is talking about the judges, he added, “Fundamentally, the most important reason we do this is to find talent.”

Asked about bringing in Abdul as a judge on “X Factor,” Cowell replied: “I adore Paula. Whatever happens, I will be working with her in some capacity, because I miss her.” But Victoria Beckham, a guest judge this season on “Idol,” won’t be joining his new show, Cowell said without explanation.

Cowell apparently carefully chose his time to resign, saying he didn’t 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.”

“American Idol,” which is entering its ninth season this week, has been the country’s most popular television program for the past five years and has launched such stars as Kelly Clarkson, Carrie Underwood and Chris Daughtry.

Yet viewership for “American Idol” has been shrinking since its 2005 peak when it averaged more than 30 million weekly viewers, according to research chief Brad Adgate of Horizon Media; last year’s weekly audience averaged just under 25 million. The median age of viewers has shot up, from nearly 32 years old in the first season to about 44 last year.

Rice and Cowell said “Idol” and “The X Factor” would complement each other, not detract.

Airing the network’s talent shows throughout the season — “The X Factor” in the fall, “American Idol” from January through May and “So You Think You Can Dance” in the summer — will be a “source of strength” for Fox’s schedule, Rice said.

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AP Television Writers David Bauder in Pasadena and Frazier Moore in New York contributed to this report.