Some flights to and from the UK are facing delays because of problems affecting French air traffic control.
British Airways said an “outage” had affected flights travelling through French and Spanish airspace.
Easyjet said it was experiencing disruption due to a “partial failure of French air traffic control systems”.
Paris Airport tweeted that a “national computer failure related to the centralisation of flight plans” on Sunday morning was now resolved.
But it warned that delays are still expected.
National Air Traffic Services (NATS) said it does not know how many flights have been affected but said it is working with airlines in the UK to try to minimise disruption.
Gatwick Airport said passengers should check with airlines on the status of their flights before heading to the airport.
Easyjet said it has been forced to cancel 120 flights out of just under 2,000 scheduled to take off on Sunday.
Affected passengers were contacted directly and given the option of transferring their flight for free or receiving a refund, it said.
The airline added it was seeing significant delays and recommended all its passengers, regardless of their destination, check the status of their flight at www.easyjet.com/en/flight-tracker for real time information before going to the airport.
British Airways also urged customers to check the status of their flights online.
The airline tweeted that it expects disruption to services to France and Spain, as well as those which fly over these countries on the way to other destinations.
It said it would offer flexible rebooking options for passengers who want to change their dates of travel as a result of the disruption.
Travel expert Simon Calder said: “France is absolutely at the heart of European air traffic control – some 60% of all Easyjet flights to anywhere go over French territory.
“This appears to be some kind of malfunction which has greatly reduced the flow rate [of flights] so there’s reports of pilots in Lisbon, for example, trying to get to the UK telling passengers we could be five hours late.”
He said affected passengers will not be eligible for compensation, explaining: “It’s not the airlines’ fault.”
But he said the airlines have a strict duty of care, which means they must provide meals and if necessary accommodation to passengers.
He added: “They also have to rebook you on the first available flight, ideally on the same day, even if it means paying money to a rival to get you home.”
Please include a contact number if you are willing to speak to a BBC journalist. You can also contact us in the following ways: