Some flights to and from the UK are facing delays and cancellations due to problems affecting French airspace.
British Airways said flights heading to, or passing over, France and Spain had been affected.
EasyJet said it has been forced to cancel 180 flights out of almost 2,000 scheduled to take off on Sunday.
The French aviation regulator said a "computer failure" had affected control centres at about 01:30 BST on Sunday, but the issue had now been resolved.
In a statement posted on Twitter, it added that delays "should be reduced gradually".
National Air Traffic Services (Nats) said it does not know how many flights have been affected but it is working with airlines in the UK to try to minimise disruption.
It added that French authorities had been allowing extra flights to enter the country's airspace on Sunday afternoon to try to limit knock-on delays.
Gatwick Airport said passengers should check with airlines on the status of their flights before heading to the airport.
EasyJet said it had contacted affected passengers 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 using its online "flight tracker" tool for real time information before going to the airport.
Like all other airlines, we're impacted by an air traffic control outage which is causing delays to flights using French and Spanish airspace. We're working hard to minimise the impact on our customers' travel plans. AlexF— British Airways (@British_Airways) September 1, 2019
British Airways also urged customers to check the status of their flights online.
The airline said an air traffic control "outage" in France had hit flights going through both French and Spanish airspace.
Some passengers have told the BBC their British Airways flights had been cancelled.
The airline said it would not release any cancellation figures but added any affected customers had been notified directly.
It said it would offer flexible rebooking options for anyone who wants to change their dates of travel as a result of the disruption.
Ryanair advised customers on its website there had been a "serious French ATC [air traffic control] equipment failure" early on Sunday morning.
It said delays of "up to three hours are being suffered".
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."
The disruption is having a wider knock-on effect in the UK, with some flights from Scotland to England cancelled.
Richard Martin was due to fly from Edinburgh to London Stansted when EasyJet texted to say his flight had been cancelled.
"We are booked on another flight tomorrow but I'm due to be back at work," he said.
"The queues at the airport and everything are crazy and we've had some family members say something similar has also happened to them."