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The series - a co-production with BBC Worldwide - will be written by Man of Steel scribe David S Goyer.
Upstairs Downstairs star Haddock will play Da Vinci's lover Lucrezia Donati, reports Deadline.
Tom Riley was previously cast in the title role. Described as a "historical fantasy", Da Vinci's Demons will focus on the artist's turbulent youth in Renaissance Florence.
Haddock previously appeared in Cinemax / Sky1 action drama Strike Back and BBC sitcom How Not to Live Your Life.
Her film credits include last year's Captain America: The First Avenger and The Inbetweeners Movie.
Julie Gardner (Doctor Who) will executive produce Da Vinci's Demons alongside BBC Worldwide Productions head Jane Tranter.
The eight-part series will enter production in summer 2012 and will air on Starz in 2013.
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