A stretch of the M4 between junction 20 and junction 22 at the Prince of Wales Bridge over the River Severn as closed for several hours, as well as the eastbound section of the bridge and the M48 between the bridge and the M4.
Avon and Somerset Police have urged anyone who was travelling along that stretch of the M4 and thinks they may have valuable information or dashcam footage to get in touch.
National Highways said police started to release traffic caught on the M4 and M48 eastbound around midnight.
The police said several drivers called the police from 6.40pm on Saturday to report something on the road, according to the force.
Avon and Somerset Police said the body was believed to be that of a man in his 40s, whose next of kin have been informed.
