According to reports, the former Spice Girl could miss Saturday and Sunday’s live shows from Wembley Arena.
Mel was forced to pull out of a press event on Thursday as well as an evening of rehearsals, meaning her act, Andrea Faustini, was left without a mentor just two days before the grand finale.
It is not known what is wrong with the 39-year-old, but according to the Sun, it is described as ‘serious’.
An ‘X Factor’ spokesman told the newspaper there was a chance Mel wouldn’t be taking her seat next to Simon Cowell, Cheryl Fernandez-Versini and Louis Walsh on the judging panel.
It is believed if Mel did pull out, show bosses are considering drafting in a replacement.
A source said: “This is a real disaster.”
One option would be for former judge Gary Barlow to stand in for Mel.
The Take That star will be joined by his bandmates and all three finalists - Fleur East, Ben Haenow and Andrea Faustini - to open the show with a rendition of their hit single ‘Rule The World’.
Saturday night's 'X Factor' will also feature performances from Ed Sheeran and 'All About That Bass' singer Meghan Trainor.
Sunday's show will see Sam Smith and One Direction taking to the stage.