The 23-year-old hairdresser was hoping to boost his chances of winning the contest this weekend by appearing at the prestigious gig, but a throat infection has put him out of action.
The Mirror quoted a source as saying: "Marcus's throat is really bad and although he wanted to try to sing in the soundcheck he was told to forget it.
"He has been banned from singing for 24 hours by the doctors. They are going to assess him again on Tuesday to see if there is an improvement.
"Gary has another gig that night so he could still try to appear then but it would be a risk."
Marcus will be going head-to-head with Amelia Lily and Little Mix in the final of the ITV1 talent show - which has had its closest-fought series for years.
Get well soon from everyone at Eleven Marcus!
We told you this morning how Misha B, who got booted from the show on Sunday, has made ANOTHER famous contact in the form of Missy Elliot.