The film is being made by Working Title and Amazon and production is underway, with shooting set to take place in Shropshire, Wales and Gloucester. It is a longstanding passion project for Girls creator and star Dunham, who first announced plans to adapt Karen Cushman's 1994 children's novel into a film in 2014.
Set in 13th century England, the story follows the rebellious 14-year-old Catherine, daughter of a knight, called Birdy because of her love of birds, who resists the arranged marriages her father has planned out for her.
News of Piper and Scott's casting was broken by the Daily Mail.
Piper is returning for a second series of I Hate Suzie it was announced last month, with filming on the Sky Atlantic dark comedy drama due to begin next year.
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