Britney Spears and Demi Lovato have signed up as judges for the US version of the X Factor. The singers will join Simon Cowell and L.A. Reid on the judging panel when the show returns in the autumn. Fox confirmed earlier this year that Paula Abdul and Nicole Scherzinger would not be returning. The programme's creator Simon Cowell had promised to shake up the show after ratings failed to match his predictions. Speaking as the new judges were presented in New York, he said: "We're going to make this the best series we've ever made. "We started off well last year but we are going to seriously kick butt with the addition of these guys." 'Totally stoked' Britney Spears has sold more than 100 million albums around the world, and has had five number one singles in the UK. She said: "I'm so excited about this whole experience. It's going to be so much fun and so different from anything I've ever do. "'I'm ready to find the true star." Disney star Demi Lovato, 19, appeared in the film Camp Rock and has released three albums. Like Britney Spears, she has had personal problems since becoming famous and spent time in therapy after developing eating disorders. She has also admitted self-harming. She said she was "totally stoked" to join the show. "I'm excited to represent my generation and I'm ready to find the winner." The viewing figures for the first series of the X Factor USA fell short of the 20 million originally predicted by Simon Cowell. He recently admitted he'd been "too arrogant" and promised to revamp the programme. Cheryl Cole had been hired as a judge but was dropped two weeks into the auditions. The first series was won by 19-year-old Melanie Amaro, who got a recording contract worth more than £3 million. Fox have yet to announce a new host after Steve Jones left the show in January.