An unconfirmed number of people have been killed and as many as 43 injured in a collision involving 27 vehicles on the M5 motorway in Somerset.
Some 20 light vehicles and seven articulated lorries were involved in the smash on the motorway close to junction 25 near Taunton at around 8.30pm last night.
A spokesperson for Musgrove Park Hospital said at least 16 people were being treated for "a range of trauma injuries", some of which are said to be "life changing".
At least four people who were trapped were cut free by firefighters.
A spokesman for Devon and Somerset Fire and Rescue Service described it as "the worst road traffic collision anyone can remember".
Video footage from the scene showed several vehicles engulfed in flames and debris strewn across the road. A number of burning lorries were also on their side.
Firefighters were seen trying to pry open car doors to reach those trapped inside.
Assistant Chief Constable Anthony Bangham said a number of passengers were unable to escape from their burning cars and said crews were faced with a "massive fireball".
"Most vehicles were well alight and most continued to burn for a considerable time," he said.
"This made it very difficult to search the vehicles. Some of them have been burned to the ground."
He said it was too early to be able to establish what might have caused the accident.
One eyewitness reported a number of explosions and black smoke and others said they could feel the heat from the fires while driving on the opposite side of the motorway.
Paul O'Connor was travelling to Plymouth when the incident happened and managed to video dramatic footage of burning cars and lorries.
"I thought it was something to do with bonfire night and then realised it was something quite bad," he told Sky News.
"I have never seen anything like that - I could see people lying on the side of the road. It was quite disturbing really."
Conditions on the road were described as "difficult" at the time of the smash with reports of heavy fog and rain.
Police and 15 fire crews battled the fires and worked to free people from the wreckage on the northbound carriageway.
At least four vehicles were reported to be on fire, including two lorries.
The motorway has been closed in both directions from junctions 23 to 26 and police say it is likely to remain closed for the rest of the day.
Drivers are advised to avoid the area for the rest of the weekend.
Andrea Day from Trafficlink told Sky News it was the worst accident she has seen in the 10 years she had been working for the traffic information provider.
"I have never come across an accident of this kind - not in terms of the volume of vehicles involved," she said.
Ms Day warned of major disruption over the weekend with delays expected on the A38, the A30 and A303.
Jeremy Browne, MP for Taunton Deane, paid tribute to the "bravery" of the emergency workers who dealt with the "horrendous accident".
"There has not been a crash on this scale for many years and the implications of it will be life-changing for many people," he said in a statement.
Chief executive of Musgrove Park Hospital Jo Cubbon said: "Our thoughts are with everyone affected by this tragedy.
"I would like to pay tribute to the staff at Musgrove and all our emergency services partners who have responded with the utmost professionalism in these difficult circumstances."
Anyone concerned about friends or relatives can call an emergency number on 0800 092 0410.
Police say all other measures should be taken to make contact before this number is dialled.