Flights at Luton Airport 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 with the first commercial flights resuming 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.
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."
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. Hundreds of people were stranded, with many saying their cars were in the car park.
The airport said in a statement on Tuesday night additional staff were on hand to provide assistance to passengers. 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.
