The flying of drones at Gatwick, which has caused the runway to be closed, is “a deliberate act to disrupt the airport”, police said.
Tens of thousands of passengers are suffering travel chaos after all flights at the West Sussex airport were suspended.
The runway was closed at 9pm on Wednesday after two of the devices were seen near the airfield.
It was reopened at 3am on Thursday, but was shut again 45 minutes later after a further sighting of drones.
Police have said the drones which forced the closure of Gatwick’s runway are believed to be “industrial” models.
Gatwick said in an updated statement: “Gatwick Airport’s runway remains unavailable because of drone sightings.
“We have advised all airlines to cancel flights up to at least 4pm this afternoon, while keeping the situation under constant review.
“There is significant disruption at Gatwick and our terminals are extremely busy.
“We are prioritising the welfare of passengers during this very difficult time, and have teams across the airport looking after them as best we can.
“We anticipate disruption to continue throughout the day and into tomorrow.
“Any passengers due to fly today or tomorrow should not set off for Gatwick without checking flight information with their airline.
“We are extremely disappointed that what appears to be deliberate action is affecting journeys at this important time of year.
“We are working tirelessly with our airlines to put plans in place to recover our operation once given the go-ahead that our runway can reopen.
“Once again we apologise for the continued disruption. Safety is our absolute priority.”
At 9:15am, the airport said there was “ongoing drone activity” and the runway remained closed.
At 11:15am Gatwick said all flights remained suspended following reports of drones flying over the airfield.
Hundreds of people flying out of Edinburgh and Glasgow airports bound for Gatwick today will also be affected by the closure.
Edinburgh Airport has tweeted: “Latest from @Gatwick_Airport - please check with your airline who will have the latest information on your flight.”
Superintendent Justin Burtenshaw, of Sussex Police, said: “We believe this to be a deliberate act to disrupt the airport. However, there are absolutely no indications to suggest this is terror-related.”
More than 20 police units from two forces are searching for the perpetrator.
Mr Burtenshaw added: “Each time we believe we get close to the operator, the drone disappears; when we look to reopen the airfield, the drone reappears.”
Asked if he thought the operator would be caught, he said: “I’m convinced we will. It’s a painstaking thing with the new drones – the bigger the drone, the bigger the reach of the operator, so it’s a difficult and challenging thing to locate them, but I’ve got teams now and I’ve got investigators looking at how we do that, and I’m confident we will.”
Some 10,000 passengers were affected on Wednesday night, and a further 110,000 were due to either take off or land at the airport on 760 flights today.
Gatwick’s MP, Tory Henry Smith, wrote on Twitter: “The closure of Gatwick Airport for 12 hours now due to drone flying appears to be a deliberate criminal act with geofencing breached.”
The airport’s chief operating officer, Chris Woodroofe, blasted the “irresponsible” drone use.
He told BBC Radio 4’s Today programme that two of the gadgets had been seen flying “over the perimeter fence and into where the runway operates from”.
Mr Woodroofe added that the drones had sparked “very significant disruption for passengers” but police did not want to shoot them down because of the risk from stray bullets.
Aviation minister Baroness Sugg said: “These drones have been flown illegally and the operators, who have acted incredibly irresponsibly, could face up to five years in jail.”
Passengers faced severe disruption as 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.
Aviation website airlive.net said inbound flights were diverted to a range of UK airports as well as Amsterdam and Paris.
Lyndsey Clarke, from Southend, 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 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 on board.
The 20-year-old, from Aldershot, said: “We got told there would be some arrangements with coaches for us when we get out the terminal ... It was just chaos and they had only two coaches and taxis charging people £600 to get to Gatwick.”