The actress - who won acclaim for her portrayal of Miss Havisham in a TV version of Great Expectations - will voice the character in the animated version of Julia Donaldson's book.
Also in the cast are Rob Brydon playing a cat, while David Walliams is to appear as a frog in the half-hour film, to be screened on BBC1.
It is being made by the team behind The Gruffalo and The Gruffalo's Child, also written by Children's Laureate Donaldson and illustrator Axel Scheffler.
Room On The Broom is to be narrated by comic actor Simon Pegg and features Martin Clunes as a dog and Sally Hawkins as a bird.
In the story, a kind witch invites