Police say they are considering trying to shoot down a drone that has shut Gatwick Airport for more than 24 hours.
Sussex Police has received about 50 reports of a drone being flown around the airport since Wednesday evening (Thursday morning NZ time).
Det Ch Supt Jason Tingley said shooting the drone was discounted earlier but is a "tactical option" and something which is being "continually reviewed".
The Army has been deployed, but it was confirmed the airport would remain shut into Friday local time.
"We have got some estimations around the size of the drone, what it looks like," Det Ch Supt Tingley said.
"We haven't yet identified the specific make and model of the drone. That will influence our tactical options."
Confirming the airport's closure on Thursday evening, chief operating officer Chris Woodroofe said: "We will review overnight whether there is any potential to open tomorrow."
Tens of thousands of passengers on several airlines have been disrupted by drones flying over the airport.
In a tweet, the Ministry of Defence (MoD) said Defence Secretary Gavin Williamson had confirmed the military deployment.
He said: "The armed forces have a range of unique capabilities and this isn't something we would usually deploy but we are there to assist and do everything we can so that they are in a position to open the airport at the earliest opportunity."
Men in military uniform were seen arriving at the airport at about 20:00 GMT and liaising with police officers, before entering the grounds in convoy.
Officials from various government departments have been meeting and the prime minister is being kept informed.
Theresa May's spokesman said: "This is a serious incident, it's causing widespread disruption for passengers.
"The intention, obviously, is that it should be brought to a close as soon as possible."
Gatwick's runway has been shut since Wednesday night because devices have been repeatedly flown over the airfield.
A spokeswoman said it does not expect to re-open "for the foreseeable future and the advice to passengers remains not to come to the airport".
An Easyjet spokesperson said: "We are making every effort to get people to their destination at this important time of the year, but following reports of drones flying over Gatwick Airport, the runway remains closed and all flights are currently suspended."
The airline said it expected disruption to continue into Friday and is advising all passengers to check the status of their flight.
Passengers Carl and Lisa McCluskey, from Lincolnshire, found out from an air stewardess earlier that their flight to Egypt was delayed until further notice and they would need to check in with their airline Thomas Cook at 06:00 GMT on Friday.
Ryanair has announced all its flights operating to or from Gatwick on Friday, will instead depart from or arrive at London Stansted.
Sussex Police said the drone activity was not terror-related but was a "deliberate act" of disruption, using "industrial specification" drones.
Gatwick chief executive Stewart Wingate agreed it was a "highly targeted activity" and added: "It cannot be right that drones can close a vital part of our national infrastructure in this way. This is obviously a relatively new technology and we need to think through together the right solutions to make sure it cannot happen again."
He is still "not in a position to say when it will be safe to reopen the airport".
About 110,000 passengers on 760 flights were due to fly on Thursday, but 633 flights were cancelled.
Transport Secretary Chris Grayling said: "We'll do everything we can to ensure that if Gatwick is not open again quickly that we can get [passengers] away from other airports."
He could not confirm whether the perpetrators were close to being caught but added: "There's a huge amount of effort going on - we've got up-to-date technology, we've brought special technology into Gatwick to try and track this down."
The shutdown started just after 21:00 on Wednesday when two drones were spotted flying "over the perimeter fence and into where the runway operates from".
The runway briefly reopened at 03:01 on Thursday but was closed again about 45 minutes later due to "a further sighting of drones".
The airport said at about 12:00 a drone had been spotted "in the last hour".
Gatwick chief operating officer Chris Woodroofe said: "The police are looking for the operator and that is the way to disable the drone."
He said police had not wanted to shoot the devices down because of the risk from stray bullets.
He said it remained unsafe to reopen the airport after the drone had been spotted too close to the runway.
Mr Woodroofe added: "If we were to reopen today we will first repatriate passengers who are in the wrong place which could take several days."
The police operation
More than 20 police units from two forces are searching for the perpetrator, who could face up to five years in jail.
Supt Justin Burtenshaw, head of armed policing for Sussex and Surrey, described attempts to catch whoever was controlling the drones as "painstaking" because it was "a difficult and challenging thing to locate them".
"Each time we believe we get close to the operator, the drone disappears; when we look to reopen the airfield, the drone reappears," he said.
As a result of an appeal for information, Sussex Police said it had been inundated with calls, but urged people to contact with information "focussed on the identity or location of the drone operator".
How have passengers been affected?
About 10,000 passengers were affected overnight on Wednesday and Gatwick said 110,000 people were due to either take off or land at the airport on Thursday.
Incoming planes were diverted to other airports including London Heathrow, Luton, Birmingham, Manchester, Cardiff, Glasgow, Paris and Amsterdam.
Crowds of travellers spent the morning waiting inside Gatwick's terminal for updates, while others reported being stuck on grounded planes for hours.
A Gatwick spokeswoman said extra staff had been brought in and they were "trying their best" to provide food and water to those who needed it.
The Civil Aviation Authority said it considered this event to be an "extraordinary circumstance", and therefore airlines were not obligated to pay any financial compensation to passengers.
Alex Neill, from consumer rights group Which?, said people "may still be entitled to meals, refreshments, hotel accommodation or transfers".
Kasia Jaworska told the BBC she was travelling from Glasgow to Gatwick with her boyfriend when her flight was diverted to Luton.
She said she thought it was "strange" two drones had led to the closure of the airport.
"You would imagine there would be better security in place and emergency action for something like that," she said.
Christopher Lister, who had been returning from Kiev, posted a picture of people sleeping "on every seat and across the floors" on board his flight.
He said the photo was taken six hours after the plane - which was due to arrive at Gatwick - landed in Birmingham.
Flight from Kiev to #Gatwick was due to land last night at 21.45. We landed in Birmingham airport. Now almost 4am, still on the plane, no food or updates from our crew. Not allowed to disembark. Bodies sleeping on every seat and across the floors. ❤️✈️ #GatwickAirport pic.twitter.com/nBrPquEGFM— Christopher Lister (@Listy_cl) December 20, 2018
Luke McComiskie, whose flight ended up in Manchester, described chaotic scenes as people tried to find their way home after more than three hours stuck onboard.
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."
Whilst the airport remains closed, I and my family, along with many others, have been here for six hours. We where offered a hotel only for that to be rescinded shortly afterwards. People sleeping on floor in south terminal, utter shambles,no info #Gatwick— eddie boyes (@eddieboyes1965) December 20, 2018