Warner Bros is also said to be sounding out potential directors for the project, which will be based on the upcoming comic book from 300 creator Frank Miller.
300 helmer Zack Snyder is currently busy working on Superman reboot Man of Steel and is unavailable to make the follow-up. Jaume Collet-Serra (Unknown) and Noam Murro (Smart People) have been shortlisted for the director's job, according to Deadline.
300: Battle of Artemisia centres on Persian leader Xerxes, played by Rodrigo Santoro in 300, and how he grew into a "god-like entity". Gerard Butler will reportedly return as King Leonidas in the spinoff.
Snyder and his 300 co-writer Kurt Johnstad have penned a script for Battle of Artemisia.
300 opened in cinemas in 2007 and took more than $450 million (£280 million) at the worldwide box office.