The Scottish star has apparently passed on the role of the WikiLeaks founder's right-hand man Daniel Domscheit-Berg, the Hollywood Reporter said, because of scheduling conflicts with the X-Men sequel Days Of Future Past.
Spanish-born German actor Daniel Bruhl has now been cast in the role.
Benedict Cumberbatch is set to portray Julian, and the film, directed by The Twilight Saga: Breaking Dawn Part 2's Bill Condon, would have marked James' third reunion with the director following Atonement and Starter For Ten.
Domscheit-Berg wrote one of the books - Inside WikiLeaks: My Time With Julian Assange At The World's Most Dangerous Website - that the script for the film is based on. The movie is also based on WikiLeaks: Inside Julian Assange's War On Secrecy by David Leigh and Luke Harding.