Ryanair will likely have just 10 Boeing Max aircraft in service in time for next summer - half the number anticipated.
The carrier confirmed the news as it announced plans to closes bases in Nuremburg and Stockholm.
Summer capacity will also be cut in a number of other locations.
"We are in discussions with our people, our unions and our affected airports to finalise these reductions," a spokesman for the airline said.
The carrier also reduced its expected traffic growth for the 12 months to the end of March 2021, from 157 million passengers to 156 million, as a result of the Boeing Max delays.
The jets were grounded after two fatal crashes, the latest of which happened in March.
It remains unclear if they will be cleared to fly again.
The delays to Ryanair's Max delivery schedule were initially revealed last month.
Director of operations Neal McMahon told pilots in a memo that there was an issue with an over-wing exit on the Max 8-200 model of the jet the carrier has ordered.
He told them that the airline would be unlikely to have more than 10 Max aircraft in service for summer 2021 and indicated that base closures were likely.
The variant of the troubled jet can accommodate up to 200 seats, though the airline plans to install 197.
The aircraft also has lower fuel burn, giving it improved economies of scale.
The memo from Mr McMahon to Ryanair pilots noted that the airline was trying to secure at least 50 delivery slots for Max jets.