Thousands of air travelers in the UK and further afield are facing delays or cancellations, though UK airspace remains open. The air traffic control service later said the problem was remedied, but a backlog remained.
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."
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."
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.