More than 100,000 passengers have seen all flights from London Gatwick cancelled after two drones were spotted flying over the airport.
The first device was spotted at around 21:00 last night with a second following earlier this morning.
The runway at the second busiest airport in the UK was closed late on Wednesday in response and has yet to reopen.
Passengers have been warned disruption could last several days.
The disruption comes at the start of the festive period, when the airport had expected to welcome 2.9 million passengers.
Further drone incursions are suspected with Gatwick saying at noon that a device had been spotted “in the last hour.”
Some 760 flights were due to either arrive or depart from the airport on Thursday.
Sussex Police told passengers the incident was not thought to be terror-related but a “deliberate act” of disruption, describing the drones as of “industrial specification”.
A statement from Gatwick at 13:45 said: “Gatwick Airport’s runway remains unavailable because of drone sightings.
“We have advised all airlines to cancel flights up to at least 16:00 this afternoon, while keeping the situation under constant review.
“There is significant disruption at Gatwick and our terminals are extremely busy.
“We are prioritising the welfare of passengers during this very difficult time, and have teams across the airport looking after them as best we can.”
The airport added: “We anticipate disruption to continue throughout the day and into tomorrow.
“Any passengers due to fly today or tomorrow should not set off for Gatwick without checking flight information with their airline.
“We are extremely disappointed that what appears to be deliberate action is affecting journeys at this important time of year.
“We are working tirelessly with our airlines to put plans in place to recover our operation once given the go ahead that our runway can re-open.”
Gatwick chief operating officer Chris Woodroofe said police had not wanted to shoot the devices down because of the risk from stray bullets.
Woodroofe added: “If we were to reopen today, we will first repatriate passengers who are in the wrong place which could take several days.”
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— Gatwick Airport LGW (@Gatwick_Airport) December 20, 2018
Incoming planes were diverted to other airports including London Heathrow, Luton, Birmingham, Manchester, Cardiff, Glasgow, Paris and Amsterdam.
easyJet said it had been forced to divert a number of flights due to land at London Gatwick to other airports across the UK.
In a statement the low-cost carrier added: “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.
“Additionally, due to there being restrictions in place on the number of aircraft movements at Gatwick, there are further cancellations and delays today are expected to flights operating to and from London Gatwick.”
easyJet urged passengers to check the status of their flights before travelling to the airport.
The carrier continued: “easyJet has offered affected passengers who were diverted last night ground transportations 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, the carrier advised, while customers on cancelled flights will be entitled to a free of charge transfer to an alternative flight.
Commenting on the unfolding incident, Alex Neill, Which? managing director of home products and services, said: “This situation will understandably be frustrating for both the airlines and the tens of thousands of passengers travelling to and from Gatwick ahead of Christmas.
“While these extraordinary circumstances unfortunately mean you are not entitled to compensation, you may still be entitled to meals, refreshments, hotel accommodation or transfers.
“You don’t have to cancel your tickets though, as depending on the length of the delay, your airline should be providing you with alternative travel options or accommodation.”