33-year-old Kirby will star opposite Joaquin Phoenix as Napoleon Bonaparte in the historical epic.
Scott’s movie will focus on the French emperor’s exploits on the battlefield and in politics and also cover the passionate love between him and Josephine.
Filming on the movie is due to begin later in January but 28-year-old Comer was forced to drop out of the project due to a scheduling conflict with her theatre debut on London's West End’s Prima Facie.
The actress described having to leave Kitbag as "rubbish", telling The Hollywood Reporter's Awards Chatter podcast: "Unfortunately, due to Covid and changes of schedule, I don't think I can make Kitbag work right now due to a scheduling conflict."
Kirby was Oscar nominated for her role in Pieces of a Woman, in which she portraying a mother grieving the death of her baby daughter following her death in labour.