"Several" people were killed and up to 35 injured in the 27-vehicle crash on the M5 near Taunton, Somerset, a police official was quoted as saying. Emergency crew said the cause of the crash was not clear, but there had been reports of heavy rain and fog.
The M5 is currently shut northbound between junctions 25 and 24, and southbound between junctions 23 and 25. Some drivers reported delays of up to five-and-a-half hours and motorists have been urged to avoid the area.
Assistant Chief Constable Anthony Bangham, of Avon and Somerset Police, said "several people" had died but he could not be specific about numbers at this stage. He said: "The incident was very, very challenging and on arrival crews were faced with literally one massive fireball.
"Most vehicles were well alight and most continued to burn for a considerable time. This made it very difficult to search the vehicles. Some of them have been burned to the ground." Emergency crew said they believed the weather may have been a contributory factor in the crash, with witnesses describing having seen thick fog at the time.