"Unfortunately Mel is still unwell and following doctors advice she will not be taking part in Saturday's show," said an X Factor spokesman. "Tulisa will join the panel as a guest judge."
Mel B has been ill for several days
Tulisa, who was a judge on the show from 2011-2012 and who has featured in this year's series as a guest mentor at Louis Walsh's judges house and during Jukebox week she chose Stereo Kicks' song, will support Andrea Faustini in the final.
Mel has been ill for several days, forcing her to miss rehearsals. It is not known what is wrong with the 39-year-old, but a source told The Sun it is "serious".
Reports have circulated that the mum-of-three has been hospitalised.
"She is in hospital and is seriously ill," a source close to the star told The Mirror. "We are awaiting the result of tests and then we will know exactly what is wrong."
Tulisa has already starred in this year's X Factor with Louis Walsh
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.