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The actor is set to undergo surgery on damaged cartilage in his right knee.
The injury has made BRIT rehearsals a painful procedure, but the funny man is determined to battle through.
'My knee is killing me,' he told Xfm. 'I'm waiting for an operation but there's nothing I can do until I've finished my run of One Man, Two Guvnors at The Strand.
'I've been getting injections and I'll have to make sure I don't do any more damage on the night.'
It will be the second consecutive year in which James has hosted with an injury - last year a scatted cornea left him with blurred vision.
Nevertheless, the Gavin and Stacey star is convinced the BRIT Awards 2012 will be the best yet.
'The show is going to be incredible,' effused the 33-year-old. 'Coldplay are opening, which will be huge.
'They have spent a fortune on pyrotechnics to make it something really special.'
We will be covering all the action from tomorrow night's event, so stay tuned.