Ryanair Holdings Plc said the grounding of Boeing Co.’s 737 Max jetliner is set to slow capacity growth this winter and into next summer’s high season.

With a return date for the Max still uncertain, it has become unlikely that Boeing will be able to hand over all 58 planes that Ryanair was expecting for its peak schedule, the carrier said in a statement Tuesday.

“Boeing is hoping that a certification package will be submitted to regulators by September with a return to service shortly thereafter,” Ryanair Chief Executive Officer Michael O’Leary said in the release. “We believe it would be prudent to plan for that date to slip by some months, possibly as late as December.”

Ryanair faces a particular issue because it has ordered the high-capacity Max 200 version of the jet, which requires separate certification from U.S. and European regulators, something that could take two months beyond the baseline model’s return to service, it said.

O’Leary said last week that Ryanair would need to pare growth plans if the grounding of the Max dragged on. Boeing has been in crisis mode since regulators grounded the best-selling Max in March, after two crashes killed 346 people.