The GI Joe: The Rise Of Cobra star was in discussions in April to take on the role of a Roman soldier in director Kevin Macdonald's military drama.
Tatum will portray Marcus Aquila, a young centurion sent on a personal mission to Scotland to investigate the disappearance of his father's Ninth Legion, says Empire Online.
RocknRolla actor Mark Strong will also play a part, while Jamie Bell will portray a Celtic slave. Donald Sutherland recently joined the cast on the movie, which is currently being filmed in Hungary.
Tatum next stars in romantic drama Dear John, due out in 2010.