Flights are being delayed at Britain's two busiest airports, London's Heathrow and Gatwick, after technical problems at the main air traffic control centre for southern England.
Holidaymakers - who have already been facing delays and cancellations because of the heatwave - face more misery because of the issue - believed to be a faulty radar.
This means fewer aircraft than normal can use the two airports.
In a statement, air traffic control company Nats said: "We have a technical problem with a system at the Swanwick Air Traffic Control which is causing some flight restrictions.
"We are doing all we can to fix it as soon as possible."
Nats' European counterpart Eurocontrol said equipment issues were causing problems at Heathrow and Gatwick.
It added that the delays have reduced "but are still high".
A spokesman for Heathrow Airport said: "We are aware that NATS – the air traffic control service – is currently experiencing a technical issue with their systems that is affecting some airspace in the UK.
"Flights are currently arriving and departing at Heathrow, and we are supporting NATS to resolve the issue as quickly as possible. We apologise to passengers for any disruption that occurs as a result."
And a Gatwick Airport spokesman said: "Flights continue to arrive and depart from Gatwick however NATS - which controls UK airspace - currently has an issue with one of its radars and has put limits on the number of flights that can use Gatwick and other London airports.
"Passengers are advised to check the latest on their flight with their airline."
