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Flights departing the UK have been disrupted by an air traffic control "technical issue", the air traffic control group Nats says.
Nats says: "We are investigating a technical issue which is causing some disruption to flight departures."Our engineers are on site and we will provide an update as soon as we can.
"We apologise for the delay this will cause to journeys, passengers should check with their airline on the status of their flight."
This is a developing story - we'll have more details from airports and airlines soon.
Air traffic controller Nats says the issue with its “flight processing system” is causing flight disruption across UK airports, adding that “recovery will take some time”.
Departures from a number of airports in southern England, including Heathrow and Gatwick, have been affected by a technical problem affecting the air traffic control provider Nats.
At Heathrow, departures were initially paused around lunchtime. Traffic then resumed, before facing further stop-start disruption in the afternoon.
Incoming flights have been landing as normal. Gatwick said it was currently being affected by the issue, along with other airports.
In a statement, Nats said it was investigating an issue in its flight processing system, which it said was causing some disruption to flight departures.
Engineers were on site, it said, and it's "working urgently to resume normal operations as quickly as possible".
East Midlands has also been affected by the issue, while Manchester Airport warned that some departing flights could be delayed. Passengers have been advised to contact their airlines to check the status of their flights.
British Airways says like other airlines, it is "experiencing some disruption to flights as a result of a technical issue affecting air traffic control, which is run by National Air Traffic Control Services".
"This is entirely out of our control and we're in touch with our customers whilst our teams work to get them on their way as quickly as possible," it adds.