Barks, who is currently playing Nancy in the touring production Oliver! (the role she auditioned for on I'd Do Anything), played Eponine in the West End and at the 25th Anniversary concert at the O2 Arena.
In the film the 21-year-old will star alongside Hugh Jackman as Jean Valjean, Russell Crowe as Javert, Anne Hathaway as Fantine, Eddie Redmayne as Marius, Amanda Seyfried as Cosette and Aaron Tveit as Enjolras. Original cast members including Frances Ruffelle and Colm Wilkinson will be making cameo appearances in the Tom Hooper-directed adaptation.
Barks beat off competition from a host of big names including country singer Taylor Swift, who had been heavily tipped for the role. Eponine's evil guardians the Thenandiers are rumoured to be played by Helena Bonham Carter and Sacha Baron Cohen.
Reacting to the news she said: “I can’t believe I’m going to play Eponine, a role so close to my heart, a role I played in the West End. It’s the happiest moment of my life. The moment I found out I started to shake and then I started crying. I’m still shaking now.”