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.
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.
"There is significant disruption at Gatwick today as a result of what appears to be a deliberate attempt to disrupt flights," the airport said.
"We are extremely disappointed that passengers are being affected by this, especially at such an important time of year.
"We are prioritising the welfare of those at the airport by deploying staff into our terminals to look after people as best we can.
"We are working hard with our airlines to get information to passengers but would advise anyone booked onto flights from Gatwick, or meeting arriving passengers, not to travel to the airport without checking the status of the flight with their airline or on our website first."
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 on Thursday.
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, told the Press Association: "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."
In the latest statement on the incident, a Gatwick spokeswoman said: "Flights to and from Gatwick Airport remain suspended following reports of drones flying over Gatwick's airfield last night and in the early hours of this morning.
"We are advising passengers scheduled to fly from Gatwick not to travel to the airport without checking the status of their flight with their airline this morning. We apologise for the inconvenience, but the safety of all passengers and staff is our first priority."
Dr Rob Hunter, head of flight safety at the British Airline Pilots' Association (BALPA), said: "The public needs to understand that drones are not just toys and could have catastrophic consequences if they collide with an aircraft.
"We know a lot of drones will be under people's Christmas trees and we implore them to ensure they're aware of the rules and fly their drones in a safe and sensible manner."
"These drone sightings at Gatwick are further evidence that tougher laws and enforcement are required to keep drones clear of manned flights.
"That's why we need the registration and education process in force sooner rather than later, so people flouting the law can be caught and prosecuted.
"At the same time, BALPA is also calling for the Government to consider toughening the law to create a larger no-fly zone around airports.
"We need to ensure people flying drones take responsibility for their actions and do so responsibly with the knowledge that if they endanger an aircraft they could face jail."
EasyJet is advising passengers not to travel to airports if their flight has been cancelled.
A spokeswoman for the airline said: "Like other airlines, easyJet diverted a number of flights due to land at London Gatwick to other airports across the UK.
"As a result of this, aircraft and crew remain out of position at airports away from London Gatwick, which has resulted in a number of cancellations this morning.
"We advise all customers flying to and from London Gatwick today to not travel to the airport if they are on flights which have been cancelled.
"EasyJet has offered affected passengers who were diverted last night ground transportation to reach London Gatwick and hotel accommodation was provided for those who required it.
"Passengers who arranged their own transportation will be reimbursed by easyJet. Customers on cancelled flights will be entitled to a free of charge transfer to an alternative flight."
Dozens of people were perched on seats with jackets and coats used as makeshift blankets after being stranded in the airport overnight.
Mamosta Abdulla said he was on an Iraq-bound flight on Wednesday evening before getting stuck on the tarmac for four hours and will now miss his father's memorial service.
He said: "We got here at 6pm and should have flown at 9.10pm, but we were stuck four hours on the plane with a crying baby, the child was disabled and everyone was sweating because it was so hot in there.
"They gave us hope by showing us the safety procedure and then five minutes later they say nobody is flying.
"We got given a £12 refreshment voucher each after a couple hours of waiting and that's it.
"We've had to sleep in a freezing place, on uncomfortable chairs.
"We are in Iraq with bombs going off nearby and the plane still lands! But here some drones have shut down the airport."
A Gatwick spokesman said 110,000 passengers were due to either take off or land at the airport on 760 flights on Thursday.
He was unable to state how many of these passengers had already been affected but the first wave of flights is normally the busiest time of the day.
Around 10,000 passengers were affected on Wednesday night after the runway was closed at 9.03pm.
Passengers are advised not to travel to the airport if their flight is cancelled.
Joseph Ouechen, a photographer from Morocco, was due to fly into Gatwick on Wednesday night but had his flight diverted to Paris.
After arriving at Charles de Gaulle Airport at midnight, passengers with visas for the Schengen area were taken to a hotel but those without - "about 20%" - were left in the airport to fend for themselves, he said.
"There were families with babies who couldn't get to their suitcases for their milk and stuff," he said.
"We were asking just for a favour if (airport staff) could help but they said they couldn't do anything."
Firefighters eventually crossed the border through passport control with blankets and water at 3.30am, he said.
"To be honest, I'm so tired and when the guys from the fire (service) came with the bottles and blankets I was feeling like a war, like (I was) a refugee, but I'm just flying to the UK.
"It's surreal. I was flying to the UK and now there are firemen bringing me water and blankets."
Andri Kyprianou, from Cyprus, who had been visiting London, said she saw a pregnant woman sleeping on the floor and passengers with infants spending the night in the "freezing" South Terminal.
She said she got to the airport at 12.30am for a 3am flight to Cyprus via Kiev, only to find it had been cancelled, and that the next connection in Kiev is on Sunday.
She said: "I haven't slept since yesterday morning, we are very tired. It's freezing, we are cold, having to wear all of these coats for extra blankets.
"There were pregnant women, one of them was sleeping on the floor. There were people with small babies in here overnight, we saw disabled people on chairs.
"There were young children sleeping on the floor."
She said she will have to spend a night in Kiev, but she had been told by Ukrainian Airlines that there may be a chance of an alternative connection through Tel Aviv.
"Hopefully they will arrange a hotel for us so we don't have another night in an airport," she added.
Aviation Minister Baroness Sugg said the Government was considering extending police powers to prevent drones causing airport disruption in the future.
She told BBC News: "I think it's important to be clear this is a crime, this drone is being flown illegally.
"Earlier this year we changed the law to make it illegal to fly within a kilometre of an airport and I know that police are out and trying to bring the drone down as quickly as possible.
"This is an illegal act. We are also looking to extend police powers and early in the new year we'll be looking at our next steps on that.
"The other thing we're looking at is counter-drone technology. Technology in this area is obviously moving incredibly quickly, but we need to make sure we're able to stop such activity in future."
Baroness Sugg said authorities were hoping to get Gatwick open as quickly as possible as people travel for the Christmas break.
She added: "Our priority is to get that airport open as safely as possible so that people can fly off on their Christmas breaks, or people who are coming in to visit friends and family.
"The police are working to bring the drone down, and I am confident that they will do so. "
A mother said she has suffered an "emotional disaster" after spending the night on a cold floor with her eight-year-old-daughter and three-year-old son.
Yulia Hristova, who was meant to fly to Istanbul via Kiev at 3am and has been at the airport since midnight, said: "With two kids I'm in a difficult position, I'm so tired, I'm so upset, we've had no information.
"We were standing for hours, nobody's been on the desk. It was so cold. We were sleeping on the floor, me and my children. I lost my son during the night, and a policeman brought him back.
"I was meant to be reunited with my family, my kids were so excited they didn't sleep until 6am, they were waiting to get on the plane.
"It's been an emotional disaster.
"I'm so exhausted, I don't want to stress out but it's very worrying. What's going to happen to us in Ukraine? What if we run out of money? Are the airline going to put us in a hotel?
"I want to give up right now, it's making me so anxious."
Commons Leader Andrea Leadsom has said the use of drones that have shut down Gatwick Airport is "absolutely unacceptable".
She said: "Our hearts go out to all those who are desperately trying to go on holiday or arrive in the UK via Gatwick.
"It's absolutely unacceptable. I know the police are doing all they can to catch the operators of these drones."
Speaking during business questions in the Commons, Mrs Leadsom added interfering with flights using a drone was a criminal offence carrying a sentence of up to five years imprisonment and an unlimited fine.
Gatwick's chief operating officer, Chris Woodroofe, confirmed a recent drone sighting and said disruption would continue for several days.
He told BBC News: "There are 110,000 passengers due to fly today, and the vast majority of those will see cancellations and disruption.
"We have had within the last hour another drone sighting so at this stage we are not open and I cannot tell you what time we will open.
"It was on the airport, seen by the police and corroborated. So having seen that drone that close to the runway it was unsafe to reopen.
"Realistically if we do reopen today, what the airlines will seek to do is deal with the passengers who are on site and to prepare for an operation tomorrow morning where we repatriate passengers who are in the wrong place.
"It's realistically going to take several days to recover."
— Gatwick Airport LGW (@Gatwick_Airport) December 20, 2018
Oxford University lecturer Imma Oliveras got up at 4am to catch her flight back to the United States to see her family.
She said: "When I arrived at the airport at 6am there were already lines throughout the terminal, which was very full.
“People were trying to keep calm, but we were not getting very much information. They told us the runway would reopen at 8am, but then they told us on the loudspeaker that this was not the case.
“It is frustrating, and if people are flying drones it is very irresponsible in this area. But how is it possible for them to be allowed to fly a drone in this area? If it is deliberate, I hope they get them.”
Meanwhile Ben Daniels has been trying to return to his home in Crawley with his wife after a trip to Hamburg in Germany.
He said: "We saw the disruption at the airport and that planes were being diverted, so we decided to avoid it and make our own way back.
“We have had to shell out £500 in total to get back, we can’t afford to be stranded out here waiting for a plane to turn up.
"It is extremely dangerous. I don’t think people realise that from the smallest to the biggest aircraft carrying hundreds of people, if a drone strikes the plane it could be catastrophic.
"I don’t understand the thinking behind the individual or individuals responsible.”
Have you been affected by the delays? Contact the newsroom on 01273 021383 or email news@theargus.co.uk.
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