Police have closed 25 miles of the M5 in Somerset after a car hit an electricity pole.
The motorway is closed between junctions 22 and 16 after the crash brought down power cables at around 4.20am on Friday.
The Highways Agency said the closure was expected to remain in place for some time. Diversions have been set up and motorists are being urged to avoid the area.
All access slip roads between these junctions are closed including the southbound access slip from the M49 on to the M5 south.
The Portbury Hundred road near Portishead is also shut.
Emergency repairs are due to be carried out by electricity company Southern Power.
A spokesman for Avon and Somerset Police said: "At this stage the danger from the power cable is such that it is not safe for other motorists to use the motorway.
"This is expected to cause considerable disruption to the roads in the area and drivers are advised to avoid the area if possible.
"Decisions to close the motorway are never taken lightly as police officers are aware of the impact this has on local people who are trying to get to work, drop children off at school and go about their daily business.
"However, the power cable presents a hazard to any motorists and in the interests of public safety the decision was taken to close the road."