Universal Pictures/Working Title Films confirmed the casting to Playbill.com on Jan. 31.
Barks, a finalist in "I'd Do Anything," the BBC-TV reality search to find a leading lady to play Nancy in the West End production of Oliver!, secured her first leading role in 2008 when she headed the cast of the touring version of the recent West End production of Cabaret, playing Sally Bowles opposite Wayne Sleep as the Emcee.
Barks hails from the Isle of Man, which was re-named "Isle of Sam" for a day to mark her success in the reality TV show.
Cameron Mackintosh appeared at the curtain call of the Jan. 31 performance of Oliver! at The Manchester Palace and surprised Barks, who plays Nancy on the U.K. tour, by announcing her casting in the film.
The complete cast has not been announced, but casting for most of the principal roles has been announced.
As previously reported, the cast includes Russell Crowe (Inspector Javert), Hugh Jackman (Jean Valjean), Anne Hathaway (Fantine), Eddie Redmayne (Marius), Amanda Seyfried (Cosette) and Aaron Tveit (Enjolras). Negotiations for other roles continue. Casting for the roles of Eponine's corrupt parents, the Thenardiers, is still in negotiation.
Variety reported that George Blagden will play Grantaire in the film.
Academy Award winner Tom Hooper ("The King's Speech") is directing the film, which is based on the classic Victor Hugo novel and stage musical. William Nicholson, Alain Boublil and Claude-Michel Schönberg wrote the screenplay. Herbert Kretzmer wrote the show's English lyrics.
The picture, to be released Dec. 7, 2012, is A Working Title/Cameron Mackintosh production produced by Tim Bevan, Eric Fellner, Debra Hayward and Cameron Mackintosh.
As previously reported, Colm Wilkinson, the original Jean Valjean in the 1985 London and 1987 Broadway productions of Les Misérables, will play the pivotal role of the Bishop of Digne in the new film.
The good bishop's act of kindness and mercy propels petty criminal Valjean into a life in service of humanity, setting the events of the story in motion. "I have bought your soul for God," sings the cleric.
Irish-born Wilkinson was a 1987 Best Actor Tony Award nominee for playing Valjean. It was his only Broadway appearance. He played the title role in the Toronto company of The Phantom of the Opera; that performance was captured on a cast recording.
The show's original producer (and one of the film producers) Cameron Mackintosh told Daily Mail columnist Baz Bamigboye that Frances Ruffelle, the orginal Eponine in London and on Broadway, will have a cameo in the film, playing a prostitute in the "Lovely Ladies" sequence. She won a 1987 Featured Actress Tony Award for playing Eponine.