Flight delays and cancellations are expected to hit hundreds of thousands of passengers due to a six-day strike by French air traffic controllers.
EasyJet warned that it is expected to be advised by French authorities to cancel half of its flights to and from France.
But up to 70% of the 1,400 flights a day it operates across Europe and beyond pass through French airspace and could also be disrupted or delayed.
The industrial action is due to take place from Tuesday until Sunday (June 30) with a decision expected this morning.
The full impact is expected to be outlined following a meeting of the French Directorate General for Civil Aviation.
But easyJet said: “We expect them to advise us to cancel 50% of our flights in and out of France.”
The airline said the walkout by controllers “will severely impact flights to and from France and this may also affect flights that over fly French airspace”.
British Airways said: “The level of disruption is likely to fluctuate in different parts of France at different parts of each day.
“Unfortunately this industrial action is also highly likely to lead to delays on other short-haul services which have to overfly France.
Similar industrial action a year ago forced hundreds of flights to be cancelled as movements through French airspace was cut by 50%.
French officials warned passengers to expected “heavy disruption”, adding that a minimum of 50% of scheduled flights would be maintained.
The dispute is over planned cuts between 2015 and 2019 as part of European Commission plans for a ‘single sky Europe’ which will see airspace reorganised into blocks based on traffic flows rather than national borders.
The planned walk out by ATC staff comes amid lengthy strike action on French railways which has hit both domestic and international services.
EasyJet said: “We are doing everything possible to minimise the impact to our customers. All customers on the affected flights will be informed by SMS and email, with up-to-date information on easyJet.com.
“For cancelled flights, all our customers will be offered a free of charge transfer to a new flight or a full refund.
“We strongly advise customers who wish to travel and want to rebook easyJet flights to avoid rebooking flights for any day between 24th-30th June as there is a high likelihood of further disruption that could cause further cancellations.”
A Ryanair spokesman said: “We are assessing the situation and are not yet certain of the level of disruption expected, but will advise our customers once we have further information.”