Thousands of airline passengers faced delays on Monday after Britain’s air traffic control system was hit by a breakdown that slowed takeoffs and landings across the UK on one of the busiest travel days of the year.

More than three hours after it reported the “technical issue,” flight control operator National Air Traffic Services said it had “identified and remedied” the problem and flights could begin to return to normal. But scores of flights were cancelled, and Heathrow Airport said its schedules would be “significantly disrupted” for the rest of the day.

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. It did not disclose the cause of the problem.

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

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.”

Monday was a holiday for many in the U.K. and a date when many families return from vacations before the start of the school year.

European air traffic authority Eurocontrol warned of “very high” delays, and airports both inside and outside the U.K. 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.

Dozens of flights were cancelled at Heathrow, Europe’s busiest air hub, which urged passengers “to only travel to the airport if their flight is confirmed as still operating.”

“Teams across Heathrow are working as hard as they can to minimize the knock-on impacts and assist those whose journeys have been affected,” it said in a statement.

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

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 Sky News that “the disruption will be quite severe at some airports” and some U.K.-bound flights would likely have to land in other European countries in order to reduce the flow of inbound planes.