The single will feature groups such as Blue, Atomic Kitten and Five and is the latest track to join the starting line for the seasonal chart race, with a release on December 15.
Choirboy Jack Topping and Britain's Got Talent duo Richard and Adam are also releasing songs that day with a hope of taking the festive top spot.
It is not yet known what date The X Factor winner will release their single, but bookmakers still rank it as the favourite to be at the top of the singles chart on December 25.
The release of the Big Reunion stars' single - a cover of Wizzard's I Wish It Could Be Christmas Everyday - will raise money for ITV's Text Santa appeal and means the channel's charity fundraiser will be taking on the winner of its top-rating show.
Also featuring on the track will be Liberty X, 911, Honeyz and B*Witched, and a behind-the-scenes documentary made at the recording will be screened on ITV and ITV2.
All profits will go to Text Santa, which is now in its third year and raises money and awareness for six UK-based charities including Age UK and BeatBullying.
Liz McClarnon from Atomic Kitten said: "When we started the talks for The Big Reunion this time last year, we couldn't have imagined we'd end up recording a Text Santa Christmas single. It's all for charity and we are so humbled to be a part of it."
Gareth Malone - whose Military Wives choir beat X Factor winners Little Mix to the Christmas number one in 2011 - has also suggested he would like his new act Voices to release a festive contender.
Last year a Hillsborough charity single beat X Factor winner James Arthur in the Christmas chart, but he went on to score a number one the following week.
Bookmaker William Hill installed the Big Reunion single as second favourite to clinch the Christmas number one, at 4/1, with the X Factor winner still in front at 6/5.
Jack Topping is at 6/1, while a collaboration between Susan Boyle and an old recording of Elvis Presley is 7/1.