The UK’s air traffic control said it had fixed a technical issue that was causing delays and cancellations of flights

LONDON – Thousands of airline passengers faced delays on Monday after Britain’s air traffic control system suffered a malfunction that slowed takeoffs and landings across the UK on one of the busiest travel days of the year.

More than three hours after the “technical issue” was reported, the National Aeronautics and Space Administration (NATS) said it had “identified and fixed the problem, and flights can begin to return to normal.” But dozens of flights were canceled and Heathrow said its schedules would be “significantly disrupted” for the rest of the day.

Lyudmila Hristova said she and her husband were planning to attend their niece’s wedding in Bulgaria, but their flight from Heathrow was cancelled.

“We are now looking for some information on how to arrange another flight,” she said. “It’s very difficult, they just took us out of the airport, and it was very rude. There was no information, just some leaflets and that’s it.”

NATS said the outage affected its ability to process flight plans automatically, meaning plans had to be entered manually, which is a much slower process. He did not disclose the cause of the problem.

The service said it had “implemented traffic flow restrictions to maintain safety” but UK airspace remained open.

After resolving the issue, NATS said it was “working closely with airlines and airports to manage affected flights as efficiently as possible.” Our engineers will carefully monitor system performance when we return to normal operations.

Monday was a holiday for many in the UK and a return date for many families from holidays before the start of the school year.

European air traffic authority Eurocontrol warned of “extremely significant” delays, and airports in the UK and abroad asked passengers to expect waits and cancel. Passengers who were due to fly to Britain from European airports said they were told to expect delays of several hours or more.

Dozens of flights were canceled at Heathrow, Europe’s busiest airport, urging passengers to “only travel to the airport if it is confirmed that their flight is still operating”.

“Teams across Heathrow are working as hard as they can to minimize spillovers and assist those whose flights have been affected,” it said in a statement.

British Airways said it had to make “significant changes” to its flight schedule and advised passengers booked on short-haul flights on Monday not to go to airports without checking the status of their flight.

Aviation analyst Alastair Rosenshine, a former British Airways pilot, said the air traffic system appeared to have suffered “a kind of patchy failure rather than a complete shutdown”.

He told Sky News that “the disruption will be very severe at some airports” and it is likely that some flights to the UK will have to land in other European countries in order to reduce the flow of incoming planes.