The Kick-Ass creator told CBR that publishers switching to the day-and-date digital business model could have a "disastrous" effect on the market.
"I think digital could be a useful tool, but I'm increasingly concerned for friends in retail that they're going to get shafted here," he said. "I really think day-and-date release is a disastrous idea and makes no economic sense at all to comics as a business.
"It's potentially ruinous for comic stores, and in the long term it's not going to do publishers any favours either."
Millar's Kick-Ass 2 #5 arrives in stores today alongside Kapow! Guinness World Record Special #1, a deluxe edition of his Superior title. Neither comic will be released in digital alongside the print issues.
"This decision I took not to release Kick-Ass 2 or Superior or Nemesis or any of my upcoming books digitally every month isn't based on nostalgia," Millar added. "Yeah, I've hung around comic stores since I was 13, but this isn't just because it affects the bread and butter of a huge number of friends.
"I just think it's bad for the industry as a whole and since the Kick-Ass books in particular sell very, very well I hope it draws attention to the problem and encourages other creators to do the same."
Market leaders Marvel Comics and DC Comics now both operate under the day-and-date digital model.