The I'm a Celebrity King of the Jungle and his boyband bandmate will appear on the January 3 edition of the show.
Other guest duos in the New Year include Richard Madeley and DJ Sara Cox and Stacey Solomon and comic Lee Mack.
Madeley said: "Who Wants To Be A Millionaire? remains one of my favourite TV shows and I just hope my mate Sara and I raise a decent amount for our charities. I'll be playing for the Caron Keating Foundation. Caron's mother, Gloria Hunniford, runs the Foundation and if I don't do the business I expect to be summoned to Sevenoaks for a serious talking to, so wish me luck everyone."
Cox added: "I've always wanted to find out what it's like to be in the Millionaire hot seat and luckily I've got Richard to hold my hand if it all gets too much. I've got my fingers crossed that we can raise lots of cash for Centrepoint and open the New Year with a bang."
A Christmas Millionaire special airs tonight on ITV1, featuring Amir Khan and Jason Manford, Heston Blumenthal and Kate Garraway, Matt Cardle and Martin Kemp, and Fatima Whitbread and Russell Watson.
Who Wants To Be A Millionaire airs tonight at 8.30pm on ITV1.