The runway at Gatwick remains closed on Thursday morning following reports of drones flying close to the airport - despite briefly reopening overnight.
Flights in and out of the airport were suspended at about 9pm on Wednesday after two drones were sighted near the airfield.
Gatwick announced that the runway had reopened at about 3am on Thursday - but just 45 minutes later it was shut again.
In a statement, the airport said: "Following reports of two drones flying over the Gatwick Airport airfield at around 9pm, the airfield was closed from 21.03 on Wednesday 19th December to 03.01 on Thursday 20th December.
"Unfortunately a further sighting of drones in the vicinity of the airport has forced the runway to be closed again from 03.45 as we investigate the sighting alongside Sussex Police.
"We will update when we have suitable reassurance that it is appropriate to reopen the runway."
Passengers faced delays to their travels on Wednesday night as some flights were unable to leave the tarmac while others were diverted to alternative airports.
Some people reported being left stuck on planes for several hours while they waited to find out what was going on.
Gatwick advised anyone flying from the airport, or collecting someone, to check the status of their flight.
A spokeswoman added that airlines were working to provide affected passengers with hotel accommodation or transport for those whose flights were diverted.
Lyndsey Clarke, from Southend on Sea, said she was stuck on a plane for more than four hours after it was re-routed to Stansted.
The 27-year-old said passengers were then having to get taxis back to Gatwick after they were finally allowed off the aircraft.
Luke McComiskie's plane ended up in Manchester, and he described chaotic scenes as people tried to find their way home after more than three hours stuck onboard.
Passenger John Belo wrote on Twitter: “Plane should have departed an hour ago from Gatwick Airport - captain confirmed there are reports of a drone in the area… still waiting.”
— Honor Ireland (@honorireland) December 19, 2018
A a number of airlines, including British Airways, confirmed their flights had been affected. “Gatwick have stopped all arrivals and departures due to a drone, we’ve been told. Something that is not within our control,” British Airways tweeted on Wednesday evening
The Sussex airport has the busiest runway in the world. It closed for around 20 minutes following a similar incident last year, amid fears that drone owners were deliberately flying close to airports to film close encounters with landing aircraft.
The Civil Aviation Authority's 'Drone Code' says users must not fly their remote-controlled aerial devices near airports or airfields, or close to aircraft.
Endangering the safety of a plane with a drone is a criminal offence with a possible sentence of five years in prison, it warns.
A Gatwick Airport spokesperson, said: “Following reports of two drones flying over the Gatwick Airport airfield, we have had to suspend flights while this is investigated.
“We apologise to any affected passengers for this inconvenience but the safety of our passengers and staff is our foremost priority”.