Flights crossing over France, including to and from Ireland, are facing disruption during a 36-hour strike by the country's air traffic controllers.
Ryanair's Michael O'Leary said the airline had to cancel around 400 flights across Europe and the UK on Tuesday due to the industrial action.
The French Civil Aviation Authority said flight delays and last-minute cancellations cannot be ruled out. Passengers are being told to check the airline websites.
Mr O'Leary, Ryanair's chief executive, said the said the airline has had to cancel around 400 flights across Europe and the UK today as a result of the French strike.
In a video posted on Twitter this morning, Mr O'Leary said " the vast majority of these are overflights, they're not going to France".
He said they are flights going from "Britain to Spain, from Portugal to Germany, from Italy to Ireland".
"We respect the right of French ATC to strike, but if they go on strike, it should be French domestic flights or local flights in France that get cancelled."
Mr O'Leary said people flying across France are having their flights "unnecessarily cancelled because the European Commission..... will not take action". Flights crossing other countries such as Spain, Italy and Greece are protected from strike action, according to the Ryanair boss.
Aer Lingus has not cancelled any flights as a result of the strike as of Tuesday afternoon.
