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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The pair were said to have fallen on to tracks at Ealing Broadway at 18:10 BST on Tuesday.
Police said they were treating the deaths as "unexplained" and had not yet identied the victims.
Heathrow Express services were suspended, while First Great Western said there were delays of up to 90 minutes to and from Paddington.
British Transport Police said: "We were called at 6.10pm to Ealing Broadway station following reports that someone had been hit by a train.
"We attended along with the Metropolitan Police, London Ambulance Service and London Fire Brigade and discovered two people had been struck and killed by the train.
"We're still working to determine the identities of the deceased or how they came to actually be on the tracks."
The force said the train was London-bound.
Last night, commuters took to Twitter to express their concern.
Girl_about_town tweeted: "Just changed at Ealing Broadway. Rammed with firemen and policemen. Sadly two deaths, apparently."
Thomas Clements wrote: "Forensics still on the track at Ealing Broadway. Just witnessed an utterly horrific scene. Feel for the people who have to deal with it now."
Tom Kimber tweeted: "So the train I'm on has hit two people at Ealing Broadway. A sad business."
A spokesman for First Great Western said: "We are doing all we can to help the police with their investigation into this terrible incident."

The economy grew at an annualised pace of 2.1% in the fourth quarter of the year, the Commerce Department said, up from an earlier estimate of 1.9%.
But the data had little impact on shares, with the Dow Jones up 20.23 points, or 0.1%, at 20,679.55.
The S&P 500 edged up 2.17 points to 2,363.30 while the Nasdaq added 5.63 points to 5,903.18.
The biggest faller in the Nasdaq was sportswear maker Lululemon Athletica, which plunged by nearly a quarter after it warned that sales were likely to fall.
The company said it expected same-store sales to fall in the first quarter of the year, the first decline since 2009.
Shares in ConocoPhillips jumped nearly 6% after the energy company announced it was selling oil sands and western Canadian natural gas assets to Cenovus Energy for 17.7 billion Canadian dollars ($13.3bn; Â£10.6bn).
ConocoPhillips is not the first oil major to scale back its operations in the region, with both Royal Dutch Shell and Marathon Oil also having sold oil sands assets.
The high costs of extraction have made it hard for companies to make profits with oil prices remaining low.
Mel B will miss the X Factor final on Saturday night because she is too unwell.
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Two people, one believed to be a child, died when they were struck by a First Great Western train in west London.
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US stocks remained unmoved in early trade, despite an upward revision to the growth rate at the end of 2016.