Will you be watching? Cowell said in a Twitter message that Spears, 30, the world's biggest pop phenomenon of the 2000s, will join him on "X Factor," which searches for a new pop star from tens of thousands of ordinary Americans.

Spears will be joined by singer and actress Lovato, 19, to fill the judge's seats left by Paula Abdul and Nicole Scherzinger, who were both fired by Cowell after a disappointing first season in 2011.

Cowell announced the news on Twitter, sending a photo of himself with record producer Antonio "L.A." Reid, Spears and Lovato and saying "Can't believe it's finally happening. Very exciting!"

His Tweet landed moments before Fox made the official announcement in New York in a presentation to advertisers of its new 2012-13 season programming.

Spears and Lovato both appeared in person at the presentation, ending months of speculation about the replacements for the show.

Lovato found fame on Disney Channel's TV series "Sonny With a Chance" before launching a career as a pop singer with hit singles like "Skyscraper".

"Britney remains one of the biggest stars in the world, she's talented, fascinating - and I believe she knows exactly how to spot 'The X Factor'," Cowell said in a statement.
"Demi's had an amazing career in music, TV and film for someone her age. She's young, confident, and enthusiastic. I think it's really important that she speaks to our younger audience."

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