Emergency services are at the scene of the accident at Almondsbury Interchange.
Highways England says they will divert traffic at Junction 19 or the M32.
According to the Inrix traffic website, the smash is on the section of the motorway in a westerly direction.
The area has been in heavy traffic for the past two hours due to a defect at the second Severn Crossing that caused four miles of traffic on the M4.
Two lanes had to be closed while engineers carried out repairs on the Prince of Wales Bridge between England and Wales
Highways England has also warned motorists of bad weather, making driving conditions difficult.
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