X Factor bosses confirmed that they're following doctor's advice.
The judge, who's been mentoring Italian singer Andrea Faustini, has been ill for several days.
Tulisa Contostavlos has been chosen to join the panel as a stand-in. It's thought she is a natural choice as she has been a judge on the show previously in 2011-12.
She also featured in the show this year as a guest mentor at Louis Walsh's judges house and during Jukebox week she chose Stereo Kicks' song.
The former Spice Girl didn't appear at an X Factor press conference on Thursday and missed rehearsals.
The 39-year-old joined fellow judges Simon Cowell, Louis Walsh and Cheryl Fernandez-Versini this series.
She was left with only one contestant in the series after Paul Akister was eliminated in the fifth week of the live shows.
Mel B appeared at the Royal Albert Hall in London on Tuesday where 20-year-old Faustini performed in a charity concert.
Last month, there were worries that Cheryl would miss an X Factor show because of a throat infection. However she eventually appeared as normal.
In 2011, then-judge Kelly Rowland missed a live show because of illness. Her last-minute replacement was a former winner of the show Alexandra Burke.
The bookies' favourite to win this year's competition is Fleur East.
The X Factor final is on ITV on both Saturday and Sunday nights.
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Mel B will miss the X Factor final on Saturday night because she is too unwell.