The 'Collide' singer, who won the third series of the ITV competition in 2006, will join regular panellists Gary Barlow, Tulisa Contostavlos and Louis Walsh at auditions in London starting tomorrow (May 26).
Lewis will take over from Geri Halliwell, who joined the judges for try-outs in Liverpool and has been rumoured to become a permanent fixture on the show.
Speculation that Lewis would judge The X Factor emerged yesterday (May 25) in a report that also named Mel B and Rita Ora as potential future panelists.
The 'Bleeding Love' artist has recently appeared as a guest mentor on the Swedish version of The X Factor.
Guest judges are being sought for the ninth series of the UK contest after Kelly Rowland departed the show and Dannii Minogue rejected a rumoured £1m offer to return.
