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.
Original cast members Hugh Grant, Andie MacDowell, Kristin Scott Thomas, John Hannah and Rowan Atkinson will reprise their roles, reuniting for the first time in 25 years.
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.”
He added: “I can’t wait for audiences to tune in and discover what’s happening to our characters and their families 25 years on.”
Four Weddings director Mike Newell is returning to direct the new instalment, which will also feature James Fleet, David Haig, Sophie Thompson, David Bower, Robin McCaffrey, Rupert Vansittart, Simon Kunz, Sara Crowe and Timothy Walker.
It follows the success of Red Nose Day Actually, a short sequel to Love Actually, which premiered on Red Nose Day in 2017.
The new film will be screened on BBC One on Friday, 15 March.
Additional reporting by agencies
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