Gatwick’s runway has reopened after drones caused the airport to shut down for more than a day.
The airport, the UK’s second busiest, said 765 flights were scheduled for departure and arrival.
Boss Chris Woodroofe said police had not yet found the drone operator. Police said it was possible they were an environmental activist.
He said extra “mitigating measures” from the government and military had given him “confidence to reopen”.
Thousands of passengers remain stranded at Gatwick as police continue their search for those responsible for the chaos, which started on Wednesday night.
The airport could not operate while the drones were in flight in case they hit and damaged a plane.
Officers have so far failed to locate the “industrial specification” drones or their pilot and had been considering plans to shoot a device down.
But Steve Barry, assistant chief constable at Sussex Police, said they were in a “much better position today”.
He told BBC Breakfast there were a “number of lines of inquiry” into the “very malicious and criminal behaviour”, including the possibility it could have been the work of an environmental activist.
Mr Barry said a drone had last been seen at 22:00 GMT on Thursday.
Transport Secretary Chris Grayling said there was no evidence it was terror-related.
But he called it a type of disruption “we’ve not seen before” and “lessons need to be learned”.
He said the situation was “unprecedented, anywhere in the world”.
“Every possible measure will be put in place to make sure this can’t happen again,” he said.
But he added there was “no simple solution” and “you can’t fire weapons haphazardly around an airport”.
Mr Woodroofe would not be drawn on what measures would be taken if a drone was spotted again.
But he said: “Additional mitigating measures provided by government agencies and military have given me the confidence to reopen the airport.”
Gatwick said the flights planned for Friday would have about 126,000 passengers on board. About 140 flights have been cancelled.
Mr Woodroofe added: “My intention is to get those passengers to their destinations so that they can enjoy their Christmas.”
The first flights in and out of Gatwick were Norwegian Air, Easyjet and BA departures and a China Eastern Airlines arrival from Shanghai.
About 120,000 people had been due to fly since the runway closed.
Gatwick continues to advise passengers to check their flight status before turning up at the airport.
Passengers have complained of “freezing” temperatures in the south terminal, while others found themselves stuck abroad after inbound flights were either cancelled or diverted.
Dozens of passengers contacted the BBC to say uncertainty had led them to ditch their festive travel plans or spend extra money on new flights and hotel stays.
Budget airline Ryanair said it was switching all of its Gatwick flights to operate in and out of Stansted airport on Friday.
What has happened?
- Gatwick’s runway closed just after 21:00 GMT on Wednesday when two drones were spotted flying over the perimeter fence and into the airfield
- It briefly reopened at 03:01 on Thursday but was closed again about 45 minutes later due to further sightings
- Outbound flights were grounded, while incoming planes were redirected to other airports
- About 10,000 passengers were affected overnight on Wednesday
- On Thursday, police said the drone flight was “a deliberate act to disrupt the airport” but there was no evidence to suggest it was terror-related
- More than 20 police units from two forces joined the search for the perpetrator, who could face up to five years in jail
- The military, with “a range of unique capabilities”, was deployed to assist the police operation
- By Thursday night, 120,000 people had had flights cancelled and police said there had been more than 50 sightings of a drone since the runway was first closed
- On Friday morning, the airport confirmed the runway was open
- The first flights in and out of Gatwick were operated by Norwegian Air, BA and China Eastern Airlines
- Gatwick said there would be 765 flights in and out of the airport on Friday
The search for the drone
Sussex Police has been locked in a game of cat and mouse with the drone operator since the airport shutdown began.
Despite dozens of sightings, the main device, which detectives believe to have been “adapted and developed” to cause deliberate disruption, has not been found.
Det Ch Supt Jason Tingley said police were re-evaluating plans for armed officers to shoot the drone down after other methods failed.
The measure was initially dismissed over the risk posed by “stray bullets”.
Supt Justin Burtenshaw, head of armed policing for Sussex and Surrey, said finding the drone’s operator was “a difficult and challenging” prospect.
“Each time we believe we get close to the operator, the drone disappears; when we look to reopen the airfield, the drone reappears,” he said on Thursday.
What has happened to passengers?
Travellers have found themselves unable to fly in and out of Gatwick.
Thousands have been left stranded for hours inside the airport’s terminal building, resorting to sleeping on floors and benches.
Some who spoke to the BBC included a couple hoping to honeymoon in New York and a seven-year-old who had been due to fly to Lapland.
Others have found themselves stuck abroad.
Earlier this week, Gatwick predicted a “record-breaking” festive period, with tens of thousands of passengers expected most days.
Airport chief Mr Woodroofe refused to comment on the possibility of those affected by the chaos being awarded compensation.
The Civil Aviation Authority said it considered the event to be an “extraordinary circumstance”, and therefore airlines were not obligated to pay any financial compensation to passengers.
Alex Neill, from consumer rights group Which?, said people “may still be entitled to meals, refreshments, hotel accommodation or transfers”.
One passenger printed off his Easyjet boarding pass only to find an advert for a drone.
He tweeted: “You couldn’t make this up: the boarding pass for my maybe-cancelled Easyjet flight from #Gatwick tomorrow advertises… drones!”
It is illegal to fly a drone within 1km of an airport or airfield boundary and flying above 400ft (120m) – which increases the risk of a collision with a manned aircraft – is also banned.
Endangering the safety of an aircraft is a criminal offence which can carry a prison sentence of five years.
The number of aircraft incidents involving drones has grown dramatically in the past few years, as the popularity of the devices has increased.
In 2013 there were zero incidents, compared to almost 100 last year.
Mr Grayling said the government was looking to “go further” with drone-control, including considering age-limits for users.
He added: “Anyone who tries to do the same [as at Gatwick] again, should expect to go to jail for a long time.”