A drone flying close to Gatwick Airport led to the closure of the runway and forced five flights to be diverted.
An airport spokesman said the runway had been closed for two periods of nine minutes and five minutes on Sunday evening after the drone was sighted.
Easyjet said four of its flights had been diverted, while British Airways said one aircraft had been sent to Bournemouth. Other flights had to fly holding patterns as a precaution.
Sussex Police is investigating.
The airport said: “Runway operations at Gatwick were suspended between 18:10 BST and 18:19, and again from 18:36 to 18:41, resulting in a small number of go-arounds and diverts.
“Operations have resumed and the police continue to investigate.”
Passengers have told the BBC how their flights were diverted away from Gatwick.
Craig Jenkins, who was flying with Easyjet from Naples, Italy, said: “We were crossing over the Channel and it started circling.
“It did four or five circles, heading further east, before the captain said we were landing at Stansted.
“First, they said Gatwick was closed because of an incident. Then, shortly after, they said it was a drone.”
Mr Jenkins, who is from Greenwich, south-east London, said passengers were given the choice of disembarking at Stansted or waiting an hour and flying back to Gatwick.
Niamh Slatter, from Sussex, was flying home from Valencia, Spain, when her BA flight was diverted to Bournemouth.
“We were due to land 15 minutes early, but ended up circling over the south coast for a while,” she said.
“Our attempted landing at Gatwick was aborted quite late as the drone had been spotted again, so we were told that the flight was being diverted to Bournemouth Airport.”
An Easyjet spokeswoman said three flights would continue on to Gatwick, while passengers from a fourth, diverted to London Southend Airport, would be provided with coach transfers.
“While the circumstances are outside of our control, Easyjet apologises for any inconvenience caused,” she added.
In November 2016, the UK’s drone code was revised and updated to help pilots ensure they fly the gadgets safely.
The revised code turned the five main safety tips into a mnemonic, spelling drone, to make it easier to remember.
- Don’t fly near airports or airfields
- Remember to stay below 120m (400ft) and at least 50m (150ft) away from people
- Observe your drone at all times
- Never fly near aircraft
- Enjoy responsibly
Former senior air traffic controller Doug Maclean told BBC News aviation authorities had to “act on the safe side” in incidents involving drones.
“Drones are really very small. They are not designed to be spotted on air traffic radar.”
But he added: “Airports like Gatwick and Heathrow are very busy places, so there are lots of people aware of what a drone looks like.
“As soon as anyone sees anything like that, I am sure there is going to be a very instant report to air traffic control, who would then have to make a judgement on how dangerous the situation was.”
The British Airline Pilots’ Association’s flight safety specialist Steve Landells said the threat of drones flown near aircraft “must be addressed before we see a disaster”.
“We believe a collision, particularly with a helicopter, has the potential to be catastrophic,” he said.
The union has called for compulsory registration of drone users and said new technology should be considered, including a system where the drone transmits enough data for the police to track down the operator.
In April, the UK Airprox Board, which monitors near-miss incidents, said there had been five such incidents in one month – bringing the total during the previous year to 62.
This included one on the approach to Edinburgh Airport on 25 November 2016, in which a drone came within 75ft of an aircraft.
In another incident last November, a near-miss involving a passenger jet and more than one drone was reported in the UK for the first time near Heathrow Airport.
The Civil Aviation Authority said there were serious consequences for people who broke the rules when flying drones.
“Drone users have to understand that when taking to the skies they are potentially flying close to one of the busiest areas of airspace in the world.
“[It is] a complex system that brings together all manner of aircraft including passenger aeroplanes, military jets, helicopters, gliders and light aircraft,” a spokesman said.
“It is totally unacceptable to fly drones close to airports and anyone flouting the rules can face severe penalties including imprisonment.”