Adele was rumoured to be set to give a comeback performance after undergoing surgery on her throat, but her rep confirmed that she is still not well enough to sing.
Bosses at the reality show are now said to be trying to get Mariah to fill the slot.
A source told The Sun, 'It was always touch and go with Adele, but we were so hoping she'd make it. She's one of the world's biggest stars.
'But now we're talking to Mariah and hopefully that will come off.
'If not, we're not exactly short of popular acts for fans to enjoy.'