The Dublin-based airline said bookings had “notably weakened over the last ten days” thanks to uncertainty over the pandemic.
Ryanair has said it is cutting flight capacity by a fifth in September and October as travel restrictions to some European countries tighten.
The Irish airline announced the move shortly before rival easyJet confirmed it was going ahead with previously-announced plans to close bases at London Stansted, Southend and Newcastle airports from 31 August – resulting in 670 job losses.
Ryanair said its decision came after forward bookings “notably weakened over the last 10 days” amid continued uncertainty over coronavirus case rates.
It said most of the cuts would mean reductions in the frequency of flights rather than route closures.
The carrier said the 20% fall in capacity would be “heavily focused on those countries such as Spain, France and Sweden” where rising numbers of cases have led to increased travel restrictions.
Ireland, which Ryanair said continued to impose “uniquely restrictive” quarantine restrictions on many other EU countries, will also be affected to a large degree by the cuts, the airline added.
A spokesperson said: “Over the past two weeks as a number of EU countries have raised travel restrictions, forward bookings especially for business travel into September and October have been negatively affected, and it makes sense to reduce frequencies so that we tailor our capacity to demand over the next two months.”
Ryanair’s boss Michael O’Leary has been an outspoken critic of some of the quarantine measures being imposed to try to control the spread of the virus.
Announcing the capacity reductions, the airline said: “Proper testing at airports, and effective tracing (as is being conducted in Germany and Italy) is the only realistic and proportionate method of supervising safe intra-EU air travel while effectively limiting the spread of the COVID-19 virus.”
Ryanair said customers affected by the changes were being contacted to advise them of their options.
Shares in the Dublin-listed carried fell more than 7%
Ryanair resumed services in July following the lockdown and this month increased flights to 60% of its normal schedule.
Easyjet confirmed its base closures on Monday after a consultation launched in June, when the airline said it was in talks to cut up to 1,900 UK-based jobs.
It now says it has reduced the number of compulsory redundancies to be made, partly by offering improved voluntary redundancy packages as well as through options such as part-time and seasonal contracts, base transfers and unpaid leave.
However, the pilots’ union BALPA accused the airline of “walking away” from negotiations which could have further reduced the impact on its members.
Passengers affected by this month’s base closures will be contacted in coming days and offered options including transferring to another airport or receiving a full refund.
EasyJet added that UK quarantine rules had “created uncertainty for customers and an impact on demand for travel”.
Its shares fell 4.6% as wider airline stocks felt pain in the wake of Ryanair’s capacity cut.
The latest announcements come days after France was added to the list of countries from where travellers returning to the UK will have to enter 14 days of quarantine – a list that already also includes Spain.
An increase in coronavirus cases in Greece, Croatia and Turkey has prompted fears that they too could be added.