Thousands of air travelers faced delays on Monday 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 it was experiencing an unexplained “technical issue” that could delay flights on Monday, the end of a holiday weekend and one of the busiest travel days of the year.

The service said it had “applied traffic flow restrictions to maintain safety” and that engineers were working to find and fix the fault. It said 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 a “flight data processing system failure” in the U.K.... Read More: Associated Press