Ryanair says delays to deliveries of the Boeing 737 MAX mean it must scale back some of its services from this winter.
The carrier says it will consult with unions and staff about short-term cuts or even closures at loss-making and poorly-performing bases.
The Boeing model is expected to return to service before the end of the year after being grounded following the Lion Air and Ethiopian Airlines crashes.
However, an exact date for the return remains uncertain.
Ryanair CEO Michael O'Leary explained that the company has ordered Boeing MAX200s, a variant of the MAX aircraft.
He said: "Ryanair now hopes to receive its first MAX200 aircraft sometime between January and February 2020. Since Ryanair can only take delivery of between 6 to 8 new aircraft each month, we are now planning our summer 2020 schedules based on taking up to 30 B737 MAX aircraft deliveries up to end of May 2020.
"This is less than the 58 MAX aircraft Boeing originally scheduled to deliver for our summer 2020 schedule. This number could rise, or fall further, depending on when the B737 MAX actually returns to flight services."
He said the airline will now revise its next summer schedule based on the 30 aircraft rather than 58.
He added: "This shortfall in aircraft deliveries will necessitate some base cuts and closures for summer 2020, but also for the winter 2019 schedule."
Mr O'Leary said the cuts are "directly caused" by the delivery delays, stressing that the airline will work to recover from the delays in winter 2020 to "restore growth to normal levels" for 2021.
Boeing has been working on software revisions to the Boeing MAX planes in the wake of the two crashes in October 2018 and March 2019.
189 people died in the Lion Air crash off the coast of Indonesia, while 157 were killed when the Ethiopian Airlines plane came down shortly after take-off.
Boeing last month said it will not offer the 737 MAX planes for fresh certification in the US "until we have satisfied all requirements for certification of the MAX and its safe return to service".
