Thousands of air travellers around the world are facing delays after the United Kingdom's air traffic control system was hit by a breakdown that slowed takeoffs and landings across the country on one of the busiest travel days of the year.
More than three hours after it reported the "technical issue" on Monday, flight control operator National Air Traffic Services said it had "identified and remedied" the problem.
NATS said the outage had hit its ability to process flight plans automatically, meaning the plans had to be input manually, a much slower process.
The service said it had "applied traffic flow restrictions to maintain safety" and that engineers were working to find and fix the fault but UK airspace remained open.
Monday was a holiday for many in the UK and a date when many families return from holidays before the start of the school year.
After fixing the problem, NATS said it was "working closely with airlines and airports to manage the flights affected as efficiently as possible. Our engineers will be carefully monitoring the system's performance as we return to normal operations".
European air traffic authority Eurocontrol warned of "very high" delays, and airports both inside and outside the UK told passengers to expect waits and cancellations.
Passengers scheduled to fly to Britain from European airports said they were being told to expect delays of several hours or more.
Australia's Department of Foreign Affairs, through its Smartraveller social media accounts, urged Australians to check with their airline for further information.
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Heathrow, Europe's busiest air hub, said "national airspace issues" were causing disruption to flights, and advised passengers to check with their airline.
Dublin Airport said in a statement that air traffic control issues were resulting in delays and cancellations to some flights into and out of the Irish capital.
"We advise all passengers due to travel today to check the status of their flight with their airline in advance of travelling," it said.
British Airways said it had had to make "significant changes" to its schedule.
"Please do not travel to the airport without checking the status of your flight, as it may no longer be operating," It advised passengers booked on short-haul flights on Monday.
Air traffic monitoring website Flightradar24 said UK airports appeared to be "significantly" limiting departures, as arrivals continued.
Aviation analyst Alastair Rosenschein, a former BA pilot, said the air traffic system appeared to have suffered "some kind of patchy failure as opposed to a total shutdown".
He told the UK's Sky News that "the disruption will be quite severe at some airports" and some UK-bound flights will likely have to land in other European countries in order to reduce the flow of inbound planes.
