And former judge Tulisa will be replacing her.
In a dramatic twist to events, Mel B will now miss her finalist Andrea Faustini singing his heart out in the two-part finale this weekend (Saturday 13 and Sunday 14 December).
While a show representative confirms that Mel is "unwell", The Mirror reports that she was "rushed to hospital" on Friday for tests.
An X Factor spokesperson says: "Unfortunately @OfficialMelB is unwell & on doctor's advice she will not be taking part in tonight's show.
"Tonight @officialtulisa will join the panel as a Guest Judge."
The newspaper goes on to say that Tulisa was asked to step in at the last minute as the fourth judge at Wembley Arena.
A source says: "Tulisa is really happy to help out. She knows how important it is for the contestants to have someone there mentoring them, and getting behind them.
"She will be making sure that Andrea gets as much help and guidance as possible."
Tulisa, whose act Little Mix won the show in 2011, was Louis Walsh's mentor at his judges houses stage of the competition in the Bahamas this summer.
Mel's act Andrea will be competing against Simon Cowell's acts Fleur East and Ben Haenow to become the latest X Factor champion.
The X Factor final kicks off on Saturday at 8.30pm on ITV.
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