As many as 75,000 passengers across Europe face travel chaos today as the result of striking French air traffic controllers.
At least 550 flights are threatened with either delay or cancellation.
British Airways, easyJet and Ryanair are among airlines warning passengers to expect disruption.
The 35-hour walkout started at 6pm yesterday and will continue until 5am tomorrow. It is the third strike by French air traffic controllers this year.
The action comes as part of wider protests by French public servants against planned reforms to working conditions.
Airlines due to fly into France have been told to cut their flight schedules by 35%, leading to direct cancellations.
Other flights will also be affected as airlines are forced airlines to re-route around France.
BA said: “The main air traffic control trade unions in France will be taking industrial action during the evening of Wednesday 8 May and all day on Thursday 9 May.
“Yet again this will, unfortunately, bring another wave of unnecessary disruption for some customers travelling in Europe.
“Like all airlines, we have had to cancel a number of flights but will be using larger aircraft where possible, on other services to help affected customers.
“Although their industrial action is beyond our control, we are sorry for any potential disruption to your travel plans.”
EasyJet said: “Like all airlines, our flights to and from France, as well as those flying in French airspace, could be affected.
“Also flights that do not operate through France may still be affected as 65% of easyJet flights operate through French air space and may be delayed before arriving for your flight.
“We expect that there could possibly be delays and cancellations due to the industrial action, therefore we advise all customers to check the status of their flight on our flight tracker either on our mobile app or website.
“Please also be aware that public transport services will be affected by the strike. We recommend all customers allow plenty of extra time to get to the airport and consider alternative transport options where possible.”