Video: M5 crash: 'I just saw flames coming up out of the trucks'

Drivers and passengers were burned alive in a pile-up on the M5 that threatens to become the worst motorway accident in British history. The official number of dead following multiple collisions on the M5 near Bridgwater in Somerset had reached seven on Saturday night, with 51 injured, but Anthony Bangham, assistant chief constable of Avon and Somerset police, said the number killed would rise as officers searched wrecked and burnt-out vehicles.

Some of the dead remained trapped in their charred vehicles. Bangham said many of the lorries and cars smashed on the northbound carriageway were "burned literally to the ground".

Eyewitnesses reported how the crash triggered a massive fireball that could be seen miles away. One said: "People were trapped in their vehicles. I heard people screaming and children crying. There was also fuel which had spilled on to the road surface which was exploding."

Those involved claim the pile-up was triggered shortly after an Iceland truck vanished in the fog. Ciara Neno, from Weston-super-Mare, said: "A black fog came down and the Iceland truck literally disappeared. We managed to brake and miss the lorry but it was too late, the carnage had already started. All we heard was thump, thump, thump. My husband dragged people from the cars, the smell was horrendous and there were a number of explosions. We walked away but other people weren't so lucky."

The tragedy is likely to renew the debate about motorway safety. It comes just weeks after the government announced plans to raise the speed limit to 80mph. Thirty-four vehicles were involved in the accident, which happened at 8.25pm on Friday.

"I could see the flames from quite a way back," said Simon Bruford, 38, from Williton in Somerset, who was driving south at the time of the collision. "I spent 18 years in the Somerset fire service and have seen a lot of nasty things, but that was horrific."

The vicar at St Mary Magdalene church in Taunton said the town was in shock. The Rev Rod Corke added: "My heart goes out to those who have suffered as a result of this tragedy."

In 1993, 12 children and their teacher died when their minibus smashed into the back of another vehicle on the M40 in Warwickshire. Two other motorway accidents, in 1984 and 1985, each claimed 13 lives. But experts said Friday's tragedy was more comparable with a multi-vehicle collision on the M4 near Hungerford, Berkshire, in 1991, when 10 people died and 25 were injured in a 51-vehicle smash.

The stretch of the M5 where the latest tragedy occurred is not considered a traffic blackspot. Ppolice were investigating the possibility that thick, patchy fog had rapidly descended on the motorway, dramatically cutting visibility.

Weather forecasters said conditions had been misty in the area of the incident and any bonfires burning nearby could have made things worse.

"The particles bonfires release encourage fog droplets to form," said Gareth Harvey, a forecaster at MeteoGroup. "By 9pm there were weather stations in the county reporting visibility down to 100 metres. The roads would also have been wet due to an earlier deluge."

The scene was close to Bridgwater's Guy Fawkes carnival and it has been suggested that smoke from the event could have combined with the fog to reduce visibility. A nearby smallholding also had a large bonfire which was seen to be giving off smoke.

Bangham pledged a comprehensive and thorough investigation into what caused the tragedy. "Other factors come into play in the evening and we need to take a close look to see if they caused some kind of distraction," he said. He confirmed police would be talking to the organisers of the carnival as part of the inquiry.

Coalition ministers are to launch a consultation later this year with a view to introducing an 80mph limit on motorways in 2013.

But the Liberal Democrat MP for Taunton Deane, Jeremy Browne, said the accident should be not be used to shape the debate about motorway speed limits. "People talk about speed limits, 70mph or 80mph, but the crucial thing is to make a judgment about what is the safe speed limit for the conditions you find yourself in," Browne said.

"I was in Taunton last night and it was a pretty wet, dank and misty night. Visibility can be bad on that stretch of low-lying road but it is a pretty unremarkable stretch of motorway."

A huge contingent of police, fire and ambulance crews was called to the affected stretch of motorway, which was closed in both directions.

"It was a highly complex, very traumatic incident for our services and officers," Bangham said. When asked whether there were still further dead bodies at the crash scene, Bangham said: "Yes, I'm afraid there are. For a motorway incident this is just about as big as it gets."

He praised the heroism of the emergency services and members of the public, who attempted to pull people from burning vehicles. "The intensity of the fire – it was a fireball on the carriageway – made it incredibly difficult for people to approach. People did their very best."