Emergency work on the M4 has closed a stretch of the westbound carriageway.
It was shut from junctions 24 to 28 at 20:00 GMT on Tuesday so workers could assess a water leak in the Brynglas tunnels.
Gwent Police said diversions would be in place and warned there may be congestion.
A spokesman said: "Traffic will be diverted off the M4 at Magor and through Newport to get back on the M4 at Tredegar Park."
The road is expected to reopen at 06:00 GMT on Wednesday.