Long delays are expected for traffic heading towards Monmouth and the Midlands following an accident on the A449.
The road has been closed as a lorry carrying batteries and battery acid shed its load between the Raglan intersection and Monmouth at 0200 GMT.
A clear-up operation took place during the day.
It is hoped the road, closed northbound from the M4's Coldra roundabout to Monmouth, will reopen around 1800 GMT.
Gwent Police closed the road from the Coldra because they are concerned about the high volume of traffic trying to negotiate country lanes.
Drivers were being advised to continue across the M4 into England and join the M5 northbound at Almondsbury.
Police are recovering the wreckage and investigating the accident.
The Trunk Road Agency is co-ordinating the cleaning up and repairs to the carriageway before the road is reopened.