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Watch: The roof of the car park at Luton airport engulfed in flames
Flights at Luton Airport have resumed after a huge fire ripped through a terminal car park on Tuesday evening.
The blaze caused the building to suffer a "significant structural collapse". Bedfordshire Fire and Rescue Service believed the cause was accidental.
The fire started at about 20:45 BST and no serious injuries were reported.
Tens of thousands of passengers are believed to have been affected by flight delays. The first commercial flights resumed just after 15:00 BST.
Four firefighters and a member of airport staff were treated for the effects of breathing in smoke as they battled the huge blaze.
It broke out on level three of the terminal two car park and was thought to have started in a diesel car and spread rapidly.
One witnessed said he saw an explosion on the roof of the car park followed by a "flame that shot across the car park like a flamethrower".
After that, he saw cars exploding "every few minutes".
Firefighters remain on site working with the airport fire service, monitoring hot spots.
The car park is believed to hold up to 1,900 vehicles and hundreds of cars may have been damaged.
An investigation into the cause of the fire is continuing.
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 a "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.
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The fire was "rapidly developing and escalating" said Andy Hopkinson from Bedfordshire Fire and Rescue Service
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.
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?"
The airport said passengers arriving by car could now use the long and mid-stay car parks, while a temporary drop-off was established at the mid-stay car park
However, the DART shuttle remained closed and replacement buses were in operation.
Travel expert and broadcaster Simon Calder said: "I have calculated that there are between 40,000 and 50,000 people who will have their travel plans wrecked today."
Declan Dever, from Westport in Ireland, said: "It's no-one's fault – just have to grin and bear it."
He was trying to get back home for his brother's 80th birthday, after his 11:00 flight was cancelled.
The 65-year-old said he was lucky not to be in a rush but added "I feel sorry for people, I see children in there asleep on the ground, I feel sorry for that."
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," they added.
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Watch: Flames engulf car park at Luton Airport
Russell Taylor, 41, an account director, saw the flames after flying in to 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 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 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 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 additional staff were on hand to provide assistance to passengers.
Jareena Sarabatta and her husband Gian were due to to fly to Turkey to celebrate their 47th wedding anniversary.
They believed that they might be able to fly later in the evening but had not heard from their airline.
Mrs Sarabatta said: "It's no-one's fault what has happened but there should at least be someone to pick up the phone."
However, she was grateful they were not in the car park when the fire started.
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Watch: The BBC's Janine Machin shows us the aftermath of the Luton Airport car park fire
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.
The first commercial plane to arrive at the airport after the fire was a Wizz Air plane from Cardiff.
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".
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Flights resume at Luton Airport after car park fire
London Luton Airport
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