Any problem at Gatwick causes a ripple effect throughout Britain and continental Europe, particularly during a holiday period when air traffic control systems are under strain.
London Gatwick Airport was forced to suspend all flights Thursday due to drones flying over the airfield, causing misery for tens of thousands of stuck passengers just days before Christmas.
Flights into Gatwick, south of the British capital, were diverted to other airports while passengers waiting to take off faced gruelling delays.
Gatwick is the eighth-busiest airport in Europe and sits behind Mumbai as the world's busiest single-runway air hub.
A cat-and-mouse manhunt is underway to catch the drone operator.
Two drones were first spotted flying over the airport at around 2100 GMT on Wednesday.
The airfield briefly reopened at 0300 GMT on Thursday, but had to be closed again following further sightings.
"All flights to and from Gatwick are suspended due to ongoing drone activity around the airport. Unfortunately, there are significant delays and cancellations to all flights," the airport said.
"We apologise to everyone affected, but the safety of all our passengers and staff is our number one priority."
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.
More than 20 police units from two forces were searching for those responsible.
"We believe this to be a deliberate act to disrupt the airport. However, there are absolutely no indications to suggest this is terror-related," said Superintendent Justin Burtenshaw of the local Sussex Police force.
"Each time we believe we get close to the operator, the drone disappears; when we look to reopen the airfield, the drone reappears."
TRT World's Sarah Morice explains what is being done to catch those responsible.
'Everyone's trying to get home'
Chris Woodroofe, Gatwick's chief operating officer, told BBC radio that it would be dangerous to shoot at the drone due to the danger of stray bullets.
Inside the airport, weary passengers faced a grim wait to reach their destinations, with many returning home for the holidays.
Gisele Fenech, 43, who was travelling to Malta, was among those stranded.
"We're meeting family and it's my daughter's birthday today so it's gone all wrong. We've been looking forward to this for so long," she said.
"Everyone's trying to get home for Christmas."
Musab Rashid, 22, who was going to Copenhagen, said: "It's wrong, it's childish of them to do this, because it's affected more than 100,000 people."
Karin Sjostrom-Nandris, 49, was heading to Stockholm, said: "We can't really leave this queue because this seems to be the only place we could possibly find out any information. The queue looks like it's several hours long, so we could be here for some time."
Under British law, drones cannot be flown near aircraft or within a kilometre of an airport, or at an altitude of over 122 metres (400 feet). Those breaking the law could face up to five years in prison.
Prime Minister Theresa May's spokesman slammed the perpetrator.
"This behaviour is irresponsible and completely unacceptable," he told reporters.
"We feel for all the passengers who are facing so much disruption."
In parliament, members of the upper House of Lords raised the likelihood of a new wave of people getting hold of drones as presents this Christmas.
Gatwick serves more than 228 destinations in 74 countries for 45 million passengers a year.