(Reuters) - Britney Spears is leaving Fox singing contest "The X Factor," a source close to the show said on Thursday, having failed to turn her star power into ratings success in one season as a judge.

"I can confirm (the news)," said the source, who was not authorized to speak on the record, when asked about media reports of Spears' exit.

Celebrity and television news outlets gave differing reasons for the decision to quit.

People magazine quoted an unnamed source as saying that the 31-year-old wanted to concentrate on her music, while TV Guide's sources said Spears understood she was about to be axed.

Her departure leaves fellow judge and executive producer Simon Cowell searching for a big name and personality to lift the U.S. version of "The X Factor" past its NBC rival, "The Voice," in the ratings.

Spears was recruited to "The X Factor" with a reported $15 million salary after a 14-year singing career that made her one of the biggest pop stars of the 2000s. Reports say she has begun working on her eighth studio album.

Cowell now has to remake half of the panel for the second time in two years, after judge L.A. Reid announced before December's finale that he would be returning full time to his job as the head of Epic Records.

"The X Factor" is due to return for a third season in September 2013.

Cowell banked on Spears' huge fan base and a strong curiosity factor to give his show a second chance with audiences after a disappointing first season in 2011 that ended with the firing of two judges as well as host Steve Jones.

Spears took "The X Factor" gig with singer Demi Lovato, 20, in May to fill the judges' seats left by Paula Abdul and Nicole Scherzinger, who were both fired by Cowell.

But TV critics said Spears often came across as bland and boring in her role as a judge and mentor to the aspiring pop-star contestants, and the show lost some three million viewers compared to its first season.