A lorry carrying hay caught fire next to the Severn Bridge tolls on the M4, causing delays.
South Wales Fire and Rescue Service and Gwent Police said they were called to the scene at about 12:50 GMT on Sunday.
No one was injured and the motorway was closed in both directions. Police said the eastbound carriageway had since reopened.
A witness reported hearing a small explosion, and said they were delayed at the tolls as a result.
