Ryanair said it will be forced to cut the number of summer flights.
Europe's largest budget airline blames possible further delays in deliveries of Boeing 737 MAX planes.
The Irish company said the move would also impact jobs as it would close or make cuts at the some of its bases for the winter 2019 and summer 2020 schedules.
And yet, the company's CEO Michael O'Leary said: "We would intend to take all of the 135 firm aircraft we have ordered over the next five years."
Boeing's top-selling jet was grounded in March after crashes in Ethiopia and Indonesia that killed a total of 346 people.
The planemaker is working on a software fix that people close to the matter have said it hopes to present to regulators in September.
The airline said it could be as late as December before regulators clear the aircraft to return to the skies after two fatal crashes.