Singer Britney Spears arrives at the 2011 MTV Video Music Awards in Los Angeles August 28, 2011.[Photo/Agencies]

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

Spears, 30, the world's biggest pop phenomenon of the 2000s, and Lovato, 19, will fill the judge's slots left open by 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 "L.A" 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 $5 million recording contract to a new star found from tens of thousands of ordinary Americans.

"We've delivered. Rumors 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!"

Spears found fame as a 16-year-old in 1998 with her debut single "Baby One More Time" and went on to score multiple hits with songs like "Crazy", "Oops I Did It Again" and "Womanizer." She had a much-publicized personal and career meltdown in 2007, but made a comeback with her 2008 album "Circus" and a world tour.

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

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

"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," Cowell said in a statement.

Fox gave no details of their contracts, but Spears is widely reported to be getting $15 million for a single season, which would make her one of the highest-paid TV reality show judges.

Cowell launched the U.S. version of "X Factor" last September with great fanfare and boasts that it would topple aging "American Idol" as America's most-watched TV show.

But "X Factor" mustered only about 13 million viewers - around half the audience for "American Idol" - and many critics disliked its brash, noisy production and felt it added little that was new to the crowded TV singing contest field.

"The X Factor" faces further challenges in its second season following NBC's decision to air its hit singing contest "The Voice" in the fall of 2012, as well as the spring.

Cowell said on Monday that other changes were being made to the show but gave no details.

"This year we're going to seriously kick butt. I'm thrilled 'The Voice' is going to compete with us, but second's not great," the acid-tongued Briton said in New York.

Fox and Cowell have yet to announce a new host after firing little-known Briton Steve Jones in January. Cowell has said he would like two hosts - a man and a woman - but no names have yet emerged.