The film, directed by Oscar-winning screenwriter Aaron Sorkin, was in pre-production - ahead of a February (19) start date - when producers pulled the plug, according to The Hollywood Reporter.
Redmayne was on board to play Students for a Democratic Society co-founder Tom Hayden, an anti-war and civil rights activist in the 1960s who famously married Jane Fonda, in the film, based on Brett Morgen's documentary "Chicago 10: Speak Your Peace" - about the 1969 trial of seven defendants charged with conspiracy in connection with the counterculture protests at the 1968 Democratic National Convention in Chicago, Illinois.
The movie has been in development at Amblin Partners for a decade and at one point company boss Steven Spielberg was planning to direct.
Paul Greengrass and Ben Stiller also considered directing the film before Sorkin signed on in October.
The producers remain committed to the project - and Sorkin's vision - according to The Hollywood Reporter.
A statement from an Amblin representative reads: "Aaron just adapted 'To Kill A Mockingbird', which is premiering on Broadway this Thursday. He is currently evaluating his schedule and commitments to determine the best time and way to make 'The Trial of the Chicago 7'."