Stephen Frears will direct the film, which "details the legal fight between Ali and the U.S. government when the fighter became a conscientious objector to the Vietnam War."
Frank Doelger, Tracey Scoffield, Jonathan Cameron and Frears are the executive producers of the film, which features a screenplay by Shawn Slovo. The title role has yet to be cast.
Plummer, a current Oscar nominee for his performance in "Beginners," will portray Supreme Court Associate Justice John Marshall Harlan II with Langella as Chief Justice Warren Burger.
Plummer, who won a Golden Globe Award for his performance in "Beginners," also received an Oscar nomination for Best Actor in a Supporting Role. The Golden Globe winner plays Hal Fields in the film, which is written and directed by Mike Mills. In 1974, he took home his first Tony Award for his performance in Cyrano. He was also awarded the Tony for his performance in Barrymore and has been nominated for J.B., Othello, No Man's Land, King Lear and Inherit the Wind.
Langella, a three-time Tony Award winner for his performances in Seascape, Fortune's Fool and Frost/Nixon, was seen on Broadway earlier this season in Roundabout Theatre Company's production of Terence Rattigan's drama Man and Boy, directed by Maria Aitken.