The executive producer of the show is struggling over which star to sign up for the latest series, according to The Sun.
Gary Barlow and Louis Walsh are expected to return, but the final two places are still up in the air, the tabloid reports.
Tulisa Contostavlos was recently reported to have been axed from the show, while sources claim that Nicole Scherzinger has yet to sign up for a second series.
"Sharon is seriously back in contention, as is Mel - it could go down to the wire," one insider said.
"It is about getting the right chemistry on each panel and the best dynamic. Simon's focus is getting the American X Factor right, then he'll sort out the British show.
"While Louis and Gary are pretty certain to return, it is still all to play for for the women."
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Sharon Osbourne previously judged the show from 2004 to 2007, and has been spotted out with both Cowell and Louis Walsh in recent weeks.
Mel B appeared as a guest judge last year, and has reportedly held talks despite signing up for America's Got Talent.
Robbie Williams is also said to be in the running for a spot on the judging panel.
Photo gallery - 'The X Factor' tour: