They will appear alongside their partners, Jazz Culkin and Tamsin Egerton, with two additional celebrity pairings yet to be announced.
The special programme is scheduled to air on Channel 4 on Friday, 12 December, at 9pm.
Its primary aim is to raise funds for Stand Up To Cancer, a joint national fundraising campaign by Cancer Research UK and Channel 4.
Josh Hartnett and Tamsin Egerton expressed their excitement to participate, highlighting their long-standing affection for the show and the importance of supporting the charity.