Series creator Julian Fellowes wrote the film's screenplay and is producing the project.
"We are excited that photography is about to start on the long-awaited film and that the beloved main characters will be joined by such exceptional actors playing new roles unique to the movie," said Gareth Neame, Carnival Films' executive chairman, Thursday.
Collider.com reported Brian Percival was originally announced as the movie's director, but he has since been replaced by Michael Engler.
No reason was given for Percival's move from director to executive producer.
Staunton has connections to several of the cast members.
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