It is not known what is wrong with the 39-year-old, but a source told the Sun it is "serious". An X Factor spokesperson told the paper that there's a chance Mel won't be able to take her seat next to Simon Cowell, Cheryl Fernandez-Versini and Louis Walsh.
There are fears that Mel B will be forced to miss the X Factor final
It has also meant she has missed out on valuable time mentoring her act Andrea Faustini.
"Poor Andrea is feeling abandoned," a source told the Daily Star. "He is already nervous about the final. The poor lad is having to fend for himself backstage in the run-up to the biggest night of his life.
"But Mel can't help it. She is so poorly and may have to miss the final altogether."
Andrea Faustini will take on Ben Haenow and Fleur East in the X Factor final
In the event that Mel is unable to take part, show bosses are said to be considering drafting in a replacement – one option would be former X Factor judge Gary Barlow.
Take That are already appearing on the show, singing their hit track Rule The World with all three finalists. Saturday night's show will also feature performances from Ed Sheeran and Meghan Trainor, while Sam Smith and One Direction are set to take to the stage on Sunday.
This weekend will see either Andrea, Fleur East or Ben Haenow crowned the winner of The X Factor in what promises to be a nail-biting final.
All three acts have already recorded their winner's singles – with Fleur and Ben choosing One Republic's Something I Need, and Andrea opting for Whitney Houston's I Don't Know My Own Strength.