London's Gatwick Airport shut down late Wednesday while officials urgently investigated reports that two drones were flying above the airfield.
The airport suspended all flights, causing severe disruptions just days before Christmas during one of the heaviest travel times of the year.
Police and aviation authorities were still investigating early Thursday as incoming flights were diverted to other locations in Britain and nearby countries.
Passengers complained on Twitter that their flights had landed at London Heathrow, Manchester, Birmingham and other cities. Other flights were sent to France and the Netherlands.
One traveler whose flight was diverted tweeted that passengers were not being told when they could continue to their destination.
Gatwick advised travelers via Twitter to check flights scheduled for Thursday before heading to the airport. It also advised anyone planning to pick up arriving passengers to check first.
Any problem at Gatwick causes a ripple effect throughout Britain and continental Europe, particularly during a holiday period when the air traffic control system is under strain.
It is a busy airport 27 miles (43 kilometers) south of London, hosting a variety of short- and long-haul flights and serving as a major hub for the budget carrier easyJet.
Gatwick normally operates throughout the night but the number of flights is restricted because of noise limitations. The airport website says it usually handles 18 to 20 flights overnight during the winter months.
Gatwick said in a statement that it apologized for the inconvenience but had to put place safety first.
There have been occasional reports of drones nearly hitting commercial airliners in the London area in recent years.
Strong sales of small consumer drones have led to repeated warnings about a possible threat to scheduled flights.