The UK's air traffic control service on Monday said that because of a "technical issue" it had been forced to slow takeoffs and landings across the UK.
The National Air Traffic Service (NATS) also said that Britain's airspace was not closed, despite the disruptions.
"We are continuing to work hard to resolve the technical issue," NATS wrote in an update to its initial statement on Monday. "To clarify, UK airspace is not closed, we have had to apply traffic flow restrictions which ensures we can maintain safety."
Automatic processing of flight plans failed, NATS says
NATS issued two more updates on Monday afternoon, first describing the nature of the problem and subsequently saying that it had been "remedied."
"The flight planning issue affected the system's ability to automatically process flight plans, meaning that flight plans had to be procesed manually which cannot be done at the same volume, henc the requirement for traffic flow restrictions," NATS wrote. "Our priority is always to ensure that every flight in the UK remains safe and we are sincerely sorry for the disruption this is causing."
With the issue "identified and remedied," NATS said, "We are now working closely with airlines and airport 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."
Passengers told to check with airlines about departures
NATS advised passengers to "check with your airline on the status of your flight." Delays started to become apparent immediately, airports in the UK and beyond warned passengers to reckon with potential cancellations.
The disruption came on a busy travel day, with Monday a public holiday in much of the UK that also falls near the end of school summer holidays. Passengers in several European countries looking to fly back to Britain posted news of delays on social media.
News agency Reuters cited one traveler in Budapest hoping to fly to Britain as saying that he had been told to reckon with a wait of 8-12 hours.
Delays over Europe
AirNav Ireland, the Republic of Ireland's air traffic control service, said the issue was resulting in "significant delays for flights across Europe that are traveling to, from or through UK airspace."
The European equivalent Eurocontrol warned of "very high" delays because of a "flight data processing failure in the UK."
A spokesperson for Heathrow, London and western Europe's busiest airport, said Heathrow was working with NATS and other airport partners to try to minimize the impact on passengers. London's other major hub Gatwick warned that cancellations were likely.
Scottish airline Loganair had earlier said in a post on the social media site commonly known as Twitter and recently rebranded as X that there had been a network-wide failure of UK air traffic control computer systems.
Although Frankfurt Airport, Germany's busiest, did not immediately comment on the situation on Monday afternoon, its live arrivals board showed two of the next three planes scheduled to fly in from Heathrow as cancelled at the time of publication.
Similarly, Cologne/Bonn Airport's arrivals website showed a series of canceled or delayed planes from the UK, including flights from Edinburgh, Heathrow, London Stansted, and even one from Dublin in Ireland, DW reports.
