Ryanair CEO Michael O'Leary has told staff that job cuts are "unavoidable" as a number of factors impacted its growth plans for 2020.
The airline said 900 of its pilots and cabin crew could be affected, with job losses expected to hit from this winter.
This week Ryanair reported a fall in revenue, as overcapacity and a price war in Germany drove ticket prices lower.
Alongside its results Ryanair warned that it could cut back its Glasgow, Edinburgh and Belfast routes in the event of a no-deal Brexit.
Earlier this month the airline also said delays in the delivery of Boeing's 737 Max meant it would fly fewer of the aircraft than expected next year, which it said would lead to some bases being cut or closed.
In a video message to staff Mr O'Leary said the carrier currently had an excess of more than 500 pilots and about 400 flight attendants.
He said it would also need around 600 fewer people in those categories by next summer.
"We will need about 600 less pilots and cabin crew for summer 2020," Mr O'Leary said. "On top of this bad news, we already have a surplus of 500 pilots and some 400 cabin crew because resignations have dried up to effectively zero since the start of 2019.
The increased likelihood of a no-deal Brexit in October was also an issue for the airline, he said, particularly around its British and Irish routes.
He said said the airline would do its "very best to minimise job losses", but that some cuts were "simply unavoidable".
"I’m sorry to advise you that this means that we have to cut our aircraft numbers and our staffing, not just for summer 2020, but also in winter 2019," he said. "This will result in some base cuts, some base closures, and I’m very sorry to say, some job losses this winter for pilots and cabin crew."
"Over the next couple of weeks we will be doing our very best to minimise job losses, but some are simply unavoidable at this time."
Mr O'Leary said the final details of the job cuts and base closures would not be announced until late August.
In a statement trade union Fórsa said the development was a concern, and it was "watching the situation closely" as it awaited further information from the airline.
"[Fórsa] has told the airline's management that it expects to be consulted on any measures that could impact on the jobs, incomes or working conditions of union members," it said.
The airline had an average of 14,000 flight staff in the year to 31 March, according to its annual report.
Earlier this week, the airline reported a 21% drop in quarterly profit as overcapacity and a price war in Germany drove ticket prices lower.
It reported a profit after tax of €243m for the three months to 30 June, down from €309m a year earlier.