Police were called to an eastbound stretch of the route at Junction 30, Cardiff, at 4.22am today (July 3). The road was closed for about nine hours while emergency services were at the scene and to allow for an investigation to be carried out.
South Wales Police confirmed the death in a statement, adding: “The road has now reopened. The public are thanked for their patience and understanding.
“Next of kin has been updated and enquiries are on-going.”
A Welsh Ambulance Service spokesperson said: “We were called to an incident near Junction 30 of the M4 earlier today, with multiple resources sent to the scene.”
It has been reported traffic has now returned to normal.
A crash between J24 and J25 of the M4 near Newport has also caused long delays.
The emergency services were alerted to the crash at about 8am today.
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National Highways South West has warned motorists of long delays on the Prince of Wales Bridge with six and a half miles of congestion between J22 and J23A.
Traffic Wales South tweeted at lunchtime that the road remained closed westbound between J24 Coldra and J25 Caerleon.
There have been reports of congestion on surrounding roads the A48, A4810 and A449.
Motorists were being diverted to the A48 Southern Distributor Road through Newport.