The 'Under The Sun' singer will apparently join Gary Barlow at a stately home in Northamptonshire for the Judges' Houses stage of the competition.
Cole and Barlow are thought to have planned her appearance following their duet at the Queen's Diamond Jubilee concert in June. Sources claim that Simon Cowell was not told about the Girls Aloud star's return and that Cole "loved the mischief of it".
"Only Gary's intimate inner circle know," one source said. "He hasn't even discussed it with Simon or ITV bosses. It's been seriously top secret stuff.
"They're all under the impression it's going to be Robbie [Williams]. No-one even noticed the dates for the recording clashed with Robbie's new solo tour."
Cole will help the Take That singer decide the finalists in the over-25s category. Other guest mentors rumoured to be joining the remaining judges include Ne-Yo (said to be helping Pussycat Doll Nicole Scherzinger with the boys category in Dubai), Tinie Tempah (who will advise girl's category mentor Tulisa Contostavlos in the Caribbean) and Sharon Osbourne (who will join Louis Walsh in Las Vegas to assess the groups).
The remaining audition and Bootcamp stages of The X Factor will be shown over the next few weeks. The latest episode airs tonight at 8.10pm on ITV1.