Leona Lewis To Perform on American Idol

Here's a slight American Idol spoiler for this season:

Leona Lewis - winner of Great Britain's X Factor - will perform on the show.

"We always like to invite established guests onto the show to give the contestants an idea of the level they need to reach. Leona, with her incredible voice, fits the bill perfectly," said a source.

Leona Lewis is already a hit in England. Will that success translate to the States?