Gary Barlow said a planned duet with Cheryl Cole for Children In Need hangs in the balance because she is unwell.
The Take That star had been lining up an appearance with Cheryl at the Children In Need Rocks event in Manchester on Thursday.
It would be Cheryl's first performance for months in the UK, in a year which saw her unceremoniously dumped as a judge from the US version of X Factor.
But Gary said it was still not clear whether it would go ahead because the Girls Aloud singer was feeling "ill".
It is understood that organisers have been informed she is having problems with her throat.
Gary joked on Monday that he was now thinking of contingency plans for the event which he has put together.
Speaking at rehearsals in London, he said: "She's ill. We're waiting to hear. It's meant to be a surprise but that's the plan - if not, I don't know what we're going to do. I might just have to get on the piano and do a bit of Roll Out The Barrel."
The X Factor judge admitted it was hard work organising the charity event, and said he had made sure everything was in hand before the X Factor live shows began in August.
He has also brought together Chipmunk, Ed Sheeran, Miss Dynamite, Rizzle Kicks, Tulisa Contostavlos and Labrinth for this year's BBC Children In Need single, Teardrop.
Check out our slideshow of all the weekend's X Factor acts and click on who you think should be walking off with this year's title...