Sources say that Elba would play a Merlin-esque figure who trains and mentors Arthur.
Joby Harold penned the script and is producing with Akiva Goldsman and Tory Tunnell.
Plot details are under wraps with the film set to bow on July 22, 2016.
Warners had always planned to go out with a straight offer for Elba's role, but for the part of Arthur, the plan is to begin putting out test offers next week to actors who would test the first week of August.
Ritchie has been meeting with actors over the past few months, and sources say some of the thesps had expected to receive those offers soon, though their identities are still unknown.
The most recent of numerous "King Arthur" adaptations was a 2004 version starring Clive Owen and Keira Knightley. Warners had previously tried getting an "Arthur & Lancelot" pic off the ground with David Dobkin directing and Kit Harington starring, but the project never made it into production.
Elba continues his hot streak of meaty roles with a handful of pics set to bow including "No Good Deed" opposite Taraji P. Henson and "The Gunman" alongside Sean Penn. He also will appear in "Bastille Day" and voice Shere Khan in Disney's "The Jungle Book."
He is repped by WME, Headshell Management and the Artists Partnership.
Related storiesTim Burton's 1989 'Batman' Paved the Way for Superhero FranchisesRussell Crowe, Ryan Gosling Joining Shane Black's Cop Drama 'Nice Guys'Warner Bros. Circling Movie Rights to Best-Selling Novel 'The Goldfinch' 2014 Variety Media, LLC, a subsidiary of Penske Business Media; Distributed by Tribune Content Agency, LLCCopyright © 2014, The Baltimore Sun