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 on Sunday - of nine and five minutes - after the drone was sighted.
Easyjet said four of its flights were diverted, while British Airways said one aircraft was diverted to Bournemouth.
Other flights were put into 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."
Craig Jenkins, who was flying with Easyjet from Naples, Italy, said: "We were crossing over the Channel and it [the plane] started circling.
"It did four or five circles... before the captain said we were landing at Stansted.
"First, they said Gatwick was closed because of an incident. Shortly after, they said it was a drone."
Mr Jenkins, from Greenwich, south-east London, said passengers were given the choice of disembarking at Stansted or waiting an hour to fly back to Gatwick.
Niamh Slatter, from Sussex, was flying 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," 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."
Easyjet apologised for the inconvenience, saying the circumstances were "outside" of the firm's control.
How common are near misses involving drones?
The UK Airprox Board monitors near-miss incidents.
An Airprox is the official term for a situation where the distance between aircraft and their relative positions and speed were such that the safety of the aircraft may have been compromised.
There were 70 Airprox reports involving drones coming close to aircraft over the UK in 2016 - more than double the number for 2015.
There have been 33 incidents up to May 2017.
The Civil Aviation Authority recommends drones be flown at no higher than 400ft. However, the highest Airprox involving a drone was at 12,500ft.
Of the 142 Airprox incidents involving drones recorded since 2010, 40 of them were near to Heathrow. Six of them, up to May, had been near to Gatwick.
Rules on flying drones
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
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.
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."