Gatwick Airport has closed for a second night after continued drone sightings over the airfield.
Drone operators have caused major disruption at the airport in a "highly targeted" series of flights, resulting in travel chaos for more than 120,000 people just days before Christmas.
The airport will remain closed for the rest of the evening after drones had been sighted "within the last hour", its chief operating officer Chris Woodroofe said in an update shortly before 10pm on Thursday.
Sussex Police said there had been more than 50 sightings of drones near to the airport since the incident began on Wednesday, though not all have been confirmed.
The army was deployed with "specialist equipment" earlier, however flights remain grounded.
Detective Chief Superintendent Jason Tingley, of Sussex Police, told reporters that policy has changed and shooting down the drone is now an "tactical option" for police.
He added that he could not give a timeline on when the airport was likely to reopen.
Speaking to reporters outside the airport's South Terminal, he said: "We will do what we can to take that drone out of the sky and remove that disruption so we can get Gatwick back to normal.
"One of the options is to use firearms officers if that presents itself - they have been out on the ground today and that's a consideration and a tactical option that's open to us.
"There are a number of factors in terms of range, the height of the drone and the likely impact on us firing at the drone but that is a tactical option open to the gold commander who will make a decision based on the information available to them at the time."
Mr Woodroofe told passengers not to come to the airport unless they have been advised to do so by their airline.
It comes after the army was earlier called in to assist police in the hunt for the rogue pilots after the perpetrators escaped capture for more than 22 hours.
A spokesman for the airport earlier said the airfield would remain "closed until further notice" as the disruption showed no sign of ending.
Meanwhile Easyjet cancelled all of its flights to and from the West Sussex airport for the rest of the day on Thursday.
The incident led to calls for more action to tackle illegal drone use.
An emergency Whitehall meeting was held in response to the escalating crisis at the UK's second busiest airport.
Efforts to find the drones and whoever is operating them are becoming more and more desperate as flight cancellations continue to soar.
Some 20 police units attempted to zero in on the drone operator after the first sighting over Gatwick on Wednesday, but police told officials it was too risky to try to shoot down the two drones.
"Each time we believe we get close to the operator, the drone disappears. When we look to reopen the airfield, the drone reappears," said Sussex Police Superintendent Justin Burtenshaw.
Defence Secretary Gavin Williamson confirmed the military deployment this afternoon.
He said: "We will be deploying the armed forces to give them the help that they need to deal with the situation at Gatwick airport.
"Our armed forces are always there to support the civilian authorities."
He did not reveal what measures would be deployed but said the army has a "range of unique capabilities."
The runway has been closed almost constantly since two drones were spotted being flown inside the West Sussex airport's perimeter at 9pm on Wednesday.
Police described the devices as "industrial" models and are treating the incident as "a deliberate act to disrupt the airport".
Prime Minister Theresa May offered sympathies to upset travellers and said police may be given greater powers against drones in the future.
Her spokesman condemned the drone flying as "irresponsible and completely unacceptable".
Gatwick's chief executive Stewart Wingate said he is "not in a position to say when it will be safe to reopen the airport".
But he said that the disruption was a "highly targeted activity which has been designed to close the airport and bring maximum disruption in the run-up to Christmas", in a statement apologising to passengers on Thursday.
Mr Wingate said the airport is still receiving drone sightings in and around the Gatwick airfield.
He 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."
Some 110,000 people were due to either take off or land at the airport on 760 flights on Thursday. Around 10,000 passengers were affected on Wednesday night.
Disruption is expected to continue into Friday, which is one of the airport's busiest days of the year because of the Christmas getaway.
On Thursday, 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 passengers to check the status of their flight.
Transport Secretary Chris Grayling said "this is quite clearly a deliberate act" as he said the perpetrators must know they are breaking the law.
He added: "It's pretty clear that this is a fairly large drone - not the classic, plastic garden drone that you always see, this is a commercial sized drone that is clearly being operated deliberately in a way that every time Gatwick tries to reopen the runway the drones disappear."
Earlier this year, new laws came into force which ban all drones from flying within one kilometre of airport boundaries.
Drone users who flout the restriction face an unlimited fine, up to five years in prison, or both.
Mr Grayling said there was no known motive for the pilot of the "commercial" drone and it was "clearly" being flown by someone who wanted to disrupt Gatwick.
He told Sky News that night-flight restrictions will be lifted at other airports - probably those which serve London - so that "more planes can get into and out of the country".
"Apologies for the residents affected, but it's right and proper that we try and sort people's Christmases out," Mr Grayling said.
Labour's shadow transport secretary Andy McDonald accused the Government of being "too slow to act" on drones and urged ministers to "fast-track the introduction of a regulatory framework".
Passengers faced severe disruption as flights were unable to leave the tarmac at Gatwick, while many inbound flights were diverted to alternative airports as far away as Amsterdam and Paris.
Some people reported being left stuck on planes for several hours while they waited to find out what was going on.
Lyndsey Clarke, from Southend, said she was on a stationary 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."
Captain Paul Watts, Chief Pilot and T/Director of Flight Operations at the National Police Air Service, said: "The National Police Air Service has been involved in helping search the area near Gatwick Airport for the drone pilot and guide police on the ground.
“Sussex Police requested our support shortly after 9pm on Wednesday, and again this morning at 9am. So far, we have used three helicopter crews, from our NPAS Redhill, NPAS London and NPAS Benson bases.”
Additional reporting by agencies.
