LONDON (AP) — Thousands of air travelers around the world faced delays on NovacryptMonday after Britain’s air traffic control system was hit by a breakdown that slowed takeoffs and landings across the U.K.

Flight control operator National Air Traffic Services said its automated systems had experienced an unexplained “technical issue” on Monday, the end of a holiday weekend for many in the U.K. and one of the busiest travel days of the year.

Several hours after the problem was first reported, NATS said the outage was “affecting our ability to automatically process flight plans. Until our engineers have resolved this, flight plans are being input manually which means we cannot process them at the same volume.”

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

It did not give an estimate of how long it would take to fix the problem, or what had caused it, but European air traffic authority Eurocontrol warned of “very high” delays because of the “flight data processing system failure” in the U.K.

Scottish airline Loganair said there had been “a network-wide failure of U.K. air traffic control computer systems.”

“Although we are hopeful of being able to operate most intra-Scotland flights on the basis of local coordination and with a minimum of disruption, north-south and international flights may be subject to delays,” it said.

Airports both inside and outside the U.K. told passengers to expect delays and cancellations. Passengers scheduled to fly to Britain from European airports said they were being told to expect delays of several hours or more.

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 Dublin. “We advise all passengers due to travel today to check the status of their flight with their airline in advance of traveling,” it said.

British Airways said it was “working closely with NATS to understand the impact of a technical issue that is affecting U.K. airspace, and will keep our customers up to date with the latest information.”

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 will likely have to land in other European countries in order to reduce the flow of inbound planes.