Ryanair has said it will cut capacity by 20% in September and October following «notably weakened» bookings in recent days.
The airline said the drop was driven by «uncertainty over recent Covid case rates in some EU countries».
It said cuts will mostly be in flight numbers as opposed to route closures.
Ryanair said the cuts will be «heavily focused» on countries where virus rates have led to the UK and Ireland re-imposing travel restrictions.
The UK has re-imposed 14-day quarantines on travellers coming form countries including Spain, France and Sweden, while Ireland has similar travel restrictions for countries including Germany and the UK.
The airline increased flights to 60% of its normal schedule this month after resuming services in July.
But on Monday a spokesman for Ryanair said: «These capacity cuts and frequency reductions for the months of September and October are unavoidable given the recent weakness in forward bookings due to Covid restrictions in a number of EU countries.
«Any affected passengers in September received email notification earlier today advising them of their options.»
These include passengers being able to move flights, and get cash or voucher refunds, as set out under EU regulations, a spokesperson said.
Meanwhile, rival airline Easyjet has confirmed that it will close bases at Stansted, Southend and Newcastle, with the loss of 670 jobs.
There could be up to about 1,200 further job losses as Easyjet works through plans to cut staff due to the coronavirus pandemic.
Easyject began consultations on its plans to close the bases in June, after an announcement that it would cut staff numbers.
The majority of crew at those bases have opted for voluntary redundancy, and that process will start for pilots this week, a spokesperson said.
Out of the total number of all crew who have been at risk of redundancy, 93% have opted to take the voluntary package, which is «enhanced» – that is, they get more money than through compulsory redundancy.