The 91-year-old star will play the Balloon Lady, who featured as part of P L Travers' original series of children's books of the same name, according to The Hollywood Reporter.
Lansbury has joined the previously announced cast members Meryl Streep, Emily Blunt, Lin-Manuel Miranda, Dick Van Dyke, Emily Mortimer and Ben Whishaw in the sequel to the 1964 classic.
The musical is currently filming in London.
Directed and produced by Rob Marshall, the movie is based on a screenplay by David Magee, with new songs by Marc Shaiman and Scott Wittman and an original score by Shaiman.
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