Hundreds of departing flights from major British airports were disrupted on Tuesday after air traffic control provider NATS experienced a technical issue, with Heathrow, Gatwick, Manchester and Stansted among the hubs affected by cancellations and delays.
"We have identified that the issue was in our flight processing system. Our engineers are on site and working urgently to resume normal operations as quickly as possible," NATS said in a statement.
"Flights are still departing and airspace is open, but recovery will take some time."
Heathrow, Britain's biggest hub, said it was working with NATS on solving the problem.
Flight tracking website Flightradar24 said on social media that arrivals and departures were being regulated at airports in the London flight information region because of the issue.