Departures from a number of airports have been affected by a technical problem affecting air traffic control provider Nats.
At least 425 flights in or out of the UK have been cancelled, according to tracking website FlightRadar24. EasyJet and British Airways have cancelled more than 100 each.
Nats said just before 17:00 BST that its flight systems were starting to recover.
With few aircraft able to take off, it is understood landings are being restricted, leading to some flights being prevented from departing from overseas airports.
At Heathrow, departures were initially paused around lunchtime. Traffic then resumed, before facing further disruption in the afternoon. It is understood that very few flights are currently taking off. BBC News understands that no flights were going in or out of the airport at 17:00 BST.
Thirteen Heathrow-bound flights were diverted to other airports in the UK and Europe.
Passengers have been advised to contact their airlines to check the status of their flights.
Gatwick said it was currently being affected by the issue, along with other airports.
East Midlands has also been affected, 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.
Belfast's two airports have both advised passengers to check flight information. The City of Derry Airport said it had experienced some cancellations and delays.
Even if Nats resolves the issue with its flight processing system, the disruption is likely to persist due to backlogs.
Although the technical issue affected departures, arrivals were also hit as space on the ground became increasingly restricted.
As a result there is now a backlog of flights because some planes across Europe are in the wrong place.
Nats said on Tuesday afternoon that engineers were on site, and it was "working urgently to resume normal operations as quickly as possible".
The organisation apologised for the delays caused and advised passengers to check the status of their flight with their airline. Nats said it would provide updates on www.nats.aero and X.
Transport Secretary Heidi Alexander said she was aware of the situation, and realised it would be frustrating for passengers.
Reporting synthesized and verified under Nexvoro.tech editorial guidelines. Full primary records referenced via BBC Business.
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