Britain has sent troops to its second-biggest airport after an unprecedented attempt to cripple Christmas travel with large drones forced all flights to be cancelled.

As thousands of passengers waited at Gatwick Airport, south of London, on Thursday police hunted unsuccessfully for the operators of the large drones that reappeared near the airfield every time the airport tried to reopen the runway.

Police said there was no indication of a terrorism motive behind the devices, which first appeared on Wednesday night.

"We will be deploying the armed forces," Defence Minister Gavin Williamson told reporters.

"We are there to assist and do everything we can."

Europe's air traffic control agency Eurocontrol said the airport would remain closed until 6am GMT (2pm WST) on Friday.

Drones were seen as recently 10pm on Thursday, a Gatwick spokesman said, more than 24 hours after their first sighting.

The airport said flights would remain shut down for the rest of the evening on a day when 115,000 people were scheduled to pass through, many on the way to seasonal breaks.

Prime Minister Theresa May's spokesman condemned the stand-off as "irresponsible and completely unacceptable".

Flights were halted at 9.03pm on Wednesday after two drones were spotted near the airfield, triggering the biggest disruption at Gatwick since a volcanic ash cloud in 2010.

Police said more than 20 units were hunting the operators near Gatwick airport, 50km south of London.

Reactions from passengers stranded at London's Gatwick airport which has been forced to suspend all flights due to drones flying over the airfield, causing misery for tens of thousands of stuck passengers just days before Christmas.

Transport minister Chris Grayling said it was clearly a deliberate act.

"This is a commercial-sized drone," he said.

"Every time Gatwick tries to reopen the runway, the drones reappear."

Grayling temporarily lifted night-flying restrictions at other airports to ease congestion caused by diverted aircraft, Sky News reported.

With a surge in public enthusiasm for drones, there has been an increase in near-collisions by unmanned aircraft and commercial jets in recent years.

Richard Parker, head of air traffic management technology firm Altitude Angel, said this was the first time a major airport had been hit by such a sustained and deliberate incursion into its airspace.

Gatwick's chief operating officer Chris Woodroofe described one of the drones as a heavy industrial model.

"The police advice is that it would be dangerous to seek to shoot the drone down because of what may happen to the stray bullets," he told BBC radio.

It's illegal to fly drones within 1km of a British airport boundary, punishable by five years in prison.

Even after Gatwick reopens, the backlog and disruption are expected to last for days.

Gatwick said it was working with its airlines on recovery plans once the runway reopens, with safety its "foremost priority".

Gatwick, which competes with Europe's busiest airport Heathrow, west of London, had previously said Sunday would be its busiest day of the festive period.