United Airlines Holdings Inc. has told Boeing Co. to stop building 737 Max 10 jets for the carrier, opting to switch to a smaller variant and the rival Airbus SE A321 until the US planemaker can pull the stretched single-aisle through its long-delayed certification.

“We’ve asked Boeing to stop building Max 10s, which they’ve done, for us and start building Max 9s,” United Chief Executive Officer Scott Kirby said Tuesday at a JPMorgan investor conference. “It’s impossible to say when the Max 10 is going to get certified.”

Once the Max 10 gets clearance to operate, United will switch back to the Max 10, Kirby said. The United CEO confirmed earlier Bloomberg reports that the airline is looking to swap out some of its massive order for 277 of that variant to use the A321 instead, offering the European planemaker the rare opportunity to seize an important piece of business from its chief rival.