France, Sweden, Spain and Ireland will all be affected.
Ryanair has slashed a fifth of its flights for September and October after a fall in bookings due to the COVID-19 pandemic, the firm said Monday.
It had previously planned to run 70% of its services in September.
The cancellations will impact France, Spain and Sweden, as well as Ireland, Ryanair said, criticising countries that impose quarantine on countries "which have lower COVID-19 case rates over the last 14 days than Ireland", such as Germany and the UK.
"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,” the spokesperson said.
Passengers who were due to travel in September have been informed while those due to travel in October will be informed later today.
Ryanair had previously declared net losses for €185 million after a 99% drop in passengers between April and June, and it forecasts a similar drop for the July-September quarter of the year.
It also announced 3,000 job cuts - 15% of its workforce - due to the coronavirus pandemic, amid a heavily impacted aviation sector which already saw thousands of cuts across Europe because of COVID-19.
Learn more about