The actresses both star in the short film, called One Red Nose Day And A Wedding, which will air during the charity broadcast on Friday night.
Red Nose Day co-founder Richard Curtis, who wrote the original film and has scripted the short, said: “I am delighted to finally be able to reveal that Lily James and Alicia Vikander will appear as our very special guests in One Red Nose Day And A Wedding.”
It follows the success of Red Nose Day Actually, a short sequel to Love Actually, which premiered on Red Nose Day in 2017.
Lily James and Alicia Vikander are set to appear in a Four Weddings and a Funeral sequel for Comic Relief.