All flights at Luton Airport have been suspended until the afternoon after a huge fire ripped through a terminal car park on Tuesday evening.
Flights were halted until 15:00 BST after the fire at the multi-storey caused the building to suffer a "significant structural collapse".
Bedfordshire Fire and Rescue Service believes the blaze was accidental and started in a vehicle at about 20:45 BST.
No serious injuries have been reported.
Four firefighters and a member of airport staff were treated for the effects of breathing in smoke.
One witness said he saw an explosion on the roof of the car park followed by a "flame that shot across the car park like a flame thrower".
After that, he saw cars exploding "every few minutes".
The airport said its priority was the safety of passengers and staff and it hoped to "get the airport operational as soon as possible".
The car park is believed to hold up to 1,900 vehicles and hundreds of cars may have been damaged.
Andy Hopkinson, Bedfordshire's chief fire officer, said the service had "no intelligence than to suggest it was anything other than an accidental fire".
He said it was thought the fire started in "diesel-powered" car and then spread through the building.
The car park did not appear to have sprinklers, according to Mr Hopkinson, and he said a recommendation for sprinklers in any redevelopment would be made to the airport.
He explained to reporters that the car park's open sides would have allowed the fire to spread "horizontally" before it went up through the building.
Firefighters were staying on site to monitor the "smouldering remains".
A ramp would be installed on the unaffected part of the car park to help remove unaffected vehicles, he added.
He said: "There is a substantial number that are not damaged and our focus as well is can we remove those vehicles safely without causing any danger to the responders?"
Bedfordshire Police asked people not to travel to the area.
Public transport running to the airport has been cancelled and some roads in the immediate vicinity, such as Vauxhall Way, have been closed.
There is a heavy police presence with many officers trying to direct people away from the scene. For some stranded passengers, English is not their first language.
Many were struggling with their luggage to Luton Airport's railway station as they tried to catch a train, with others trying to get coaches to other airports in order to catch alternative flights.
All the airport hotels were fully booked and many passengers said it felt like airlines had simply dumped them.
London Luton is the UK's fifth largest airport after Heathrow, Gatwick, Manchester and Stansted, carrying more than 13 million passengers in 2022.
The region's ambulance service said a critical incident was stood down but it would "remain on scene to support fire and rescue colleagues".
Bedfordshire Fire and Rescue Service said 15 engines were sent to the airport.
A passenger who was on board a plane that was due to take off as the fire broke out said: "We were all just told to get off the flight, that there was an incident and then we were left in the airport with no proper explanation."
Two hours later, they were told there was a major incident and that they would need to leave the airport.
"It was all a little bit confusing, because I don't think the staff knew exactly what they were supposed to be doing."
Another passenger said "some of us were very frightened because we'd travelled by ourselves" and "didn't have anywhere to go" after being told to leave the airport.
Russell Taylor, 41, an account director, saw the flames after flying in to London Luton from Edinburgh.
He told the PA news agency: "There were a couple of fire engines with a car ablaze on the upper floor of the car park at just after 21:00
"A few minutes later most of the upper floor was alight, car alarms were going off with loud explosions from cars going up in flames."
Hundreds of people were stranded, with many saying their cars were in the car park.
Helen Jocelyne, from Exmouth in Devon, was returning to Luton Airport from Burgas in Bulgaria when her plane was diverted to Stansted in Essex, an hour before it was due to land.
She said a coach took her to Luton, but she had to walk to the car park with her luggage.
"I don't even know if we can get our car out yet," she said.
Another passenger, Jason Harris, was supposed to be landing at Luton Airport from Egypt.
Three hours into his flight he said passengers were informed by the pilot that the fire meant they would be diverted to Bristol Airport.
He got a taxi from Bristol to Luton, provided by airline EasyJet, and he had to get a second taxi to his home in Stevenage in neighbouring Hertfordshire.
He said: "Nightmare all round, I know there's been a fire at the airport but you'd think they'd have a back road for a way out, but it can't be done."
The airport said in a statement on Tuesday night: "All flights are currently suspended as emergency services respond to a car fire that has spread in Terminal Car Park 2.
"Access to the airport is currently restricted and we ask that people do not travel to the airport at this time."
It said additional staff were on hand to provide assistance to passengers and said they should contact their airline for flight information.
The airport's website said the affected multi-storey car park was located "just a five-minute walk to the terminal entrance".
Airline provider EasyJet, based at Luton, issued a statement apologising for the inconvenience.
It added that it would be providing hotel accommodation and meals for passengers where required.
Wizz Air, a Hungarian airline which has its UK base at Luton, warned passengers to expect cancellations and disruptions.
Charlotte Vere, the Conservative minister for aviation, maritime and security, said she was "very grateful to emergency service staff who worked hard to put out the fire at Luton Airport last night and this morning."
