FOG has been blamed for a 26-vehicle crash in which at least six people died and 50 were injured, eight seriously.
Four people had to be cut out of wreckage at the crash scene on the M5, which is believed to be Britain's worst road accident in 20 years.
More than 50 people were hurt in the crash on the M5 near Taunton in Somerset, with 41 hospitalised.
Paul Slaven, of Devon and Somerset Fire and Rescue, said it was the "worst crash" anyone at the brigade could remember, adding: "I certainly can't recall a motor collision of this magnitude involving this many casualties." Wet and foggy weather conditions have been cited as a likely factor.
The motorway was also busy because a local Guy Fawkes Carnival was taking place.
The wreckage was so extensive that 15 fire engines were sent to the scene to help rescue motorists and put out the flames fuelled by exploding petrol tanks.
Chief Inspector Paul Mogg said: "All emergency services are doing as much as they can to continue the rescue and to treat those who are injured."
A huge taskforce of police, fire and ambulance crews were called to the stricken stretch of the M5, which was closed.
Firefighters who scrambled the 15 appliances to the scene battled to rescue motorists by cutting people from cars and lorries using hydraulic equipment.
Television footage also showed motorists trying to pry open vehicle doors.
Police said the motorway is expected to remain closed for at least the next 24 hours while emergency workers recover bodies still in situ and clear the debris from the accident.
Meanwhile, witnesses spoke of the severity of the pile-up.
Simon Bruford (38) from Willerton in Somerset, said: "I could see the flames from quite a way back. I spent 18 years in the Somerset fire service and have seen a lot nasty things, but that was horrific."