The veteran singer is said to have been lined up to join the final four acts for their group performance at the beginning of the show.
Stewart will sing a cut from his new festive album Merry Christmas, Baby with the group, reports The Sun.
Producers are reportedly hoping that Stewart's appearance will help boost ratings against its BBC rival Strictly Come Dancing.
An insider said: "It is a big coup to get Rod - and to get him to perform with the contestants.
"He is an absolute legend and is a big fan of the show. He jumped at the chance of being able to perform."
Union J had tweeted earlier this week: "Group ensemble this week is gonna be amazing and great fun :) so excited to perform it."
James Arthur, Christopher Maloney and Jahmene Douglas will join the boyband for the semi-final on Saturday (December 1).
Stewart previously worked with Simon Cowell on the Helping Haiti charity track 'Everybody Hurts'.
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