Katie - who is being mentored by Cheryl Cole, 27 - didn't make it to the East London 3 Mills studios until 4.45pm, after some scenes had already been shot.
‘When she did arrive, she looked pale as a ghost,' says a source. 'She had been up all night having panic attacks and crying hysterically.'
Katie later decide not to attend the Cosmopolitan Women Of The Year awards because she was still feeling rough, but she did visit the May Fair Hotel today with Cher Lloyd and the gang.
She has been receiving round-the-clock medical care as she battles a chest infection.
But if Katie, 24, continues to feel poorly, she may have to pull out of The X Factor.
'She's very fragile and we are very concerned,' the source tells the Daily Mail.
'We are considering asking her to step aside because it is really affecting the show. She would still be able to do the X Factor tour if she is strong enough. Her well-being is our priority.'
The X Factor finalists' version of Heroes will be released on 21 November to raise money for Help For Heroes.
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