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Cowell unveils Britney as X Factor US judge

Britney Spears and former Disney Channel star Demi Lovato have officially joined the US version of The X Factor, leading creator Simon Cowell to pledge the second season would "seriously kick butt" when it returns in September.

Spears (30) and Lovato (19) will replace Paula Abdul and Nicole Scherzinger, who were both fired by Cowell after a disappointing first season in 2011.

Spears and Lovato appeared with Cowell and fellow judge Antonio 'LA' Reid at Fox television's presentation of its new 2012-13 season programming to advertisers in New York.

The announcement ended months of speculation over the new faces on the singing contest, which offers a $5m recording contract to a new star found from tens of thousands of ordinary Americans.

"We've delivered. Rumours are one thing but we've delivered," a delighted Cowell said.

Moments earlier he tweeted a photo of himself with record producer Reid, Spears and Lovato, saying "Can't believe it's finally happening. Very exciting!"

"I'm so excited by this whole experience. It's so different from anything I've ever done," Spears said in New York.

Spears is widely reported to be getting $15m for a single season, which would make her one of the highest-paid TV reality show judges.