James, 24, will face Australian actress Cate Blanchett, 43, who will play Cinderella's evil stepmother.
Scottish actor Richard Madden, 26, is listed on the Internet Movie Database as rumored to play Prince Charming.
Kenneth Branagh will direct the film, to be released next year.
Emma Watson, 23, who played Hermione Granger in the "Harry Potter" film series, had been in talks for the princess role, but a deal could not be worked out, The Hollywood Reporter and Variety reported.
James plays the rebellious Lady Rose McClare in the British drama "Downton Abbey," airing in the United States on PBS.
The "Cinderella" project was started by Disney in May 2010 after the studio's success with the computer-animated and live-action "Alice in Wonderland" fantasy film starring Mia Wasikowska and Johnny Depp, the Reporter said.