It comes after a lorry struck an overhead gantry on the southbound carriageway between junction 17 at Cribbs Causeway and junction 18 at Avonmouth at around 8.20am this morning.
Police say there have been no reported injuries.
But due to the safety risk to drivers, the M5 has been fully closed southbound and northbound.
The motorway closure is causing long delays in the area and National Highways are now urging drivers to plan ahead.
Engineers are currently assessing damage to the overhead gantry.
The incident has displaced matrix signage, severed electrical cables, while hydraulic fluid has also spilled on to the carriageway.
During the closure, southbound traffic is being diverted via the M4 westbound to the M49 to rejoin the M5 at junction 18.
While northbound traffic is being diverted with northbound traffic diverted via the M49 to M4 junction 22 to rejoin the M5.
A spokesperson for Avon and Somerset Police said: “Please be aware there are currently closures on the M5 this morning.
“We were called to the M5 at around 8.20am today following reports of a tipper truck colliding with an overhead gantry on the southbound carriageway between junctions 17 and 18 .
“The motorway is currently closed in both directions.
“There have been no reported injuries.”
Ray Morwood, South West Operations Manager for National Highways, said: “The M5 has been closed to protect the travelling public and we’re working as quickly as we can to assess the gantry damage and make safe the signage and fittings.
“The incident has occurred on a busy section of the M5, we are working with our police partners, and we appreciate this will cause significant delays to people’s journeys.
“In the meantime, we advise people to add a little extra time for their journeys, and to consider alternative routes where they can. We thank all drivers for their co-operation and patience and we’re working quickly to establish timescales for repair and recovery work.”
In the meantime, drivers can check live traffic conditions at www.trafficengland.com
