The structural safety of a tunnel which forms part of a major motorway was being assessed tonight after a lorry burst into flames inside it.
The blaze, which broke out at 8.30am, caused widespread traffic chaos on the M4 at the Brynglas tunnels.
A Welsh Government spokesman confirmed the fire had been extinguished and said that structural engineers are now assessing the amount of damage caused by the blaze.
Motorists are still being urged to avoid the busy commuter route.
No other vehicles were involved in the fire and the lorry driver escaped without injury, South Wales Fire and Rescue said.
The Government spokesman said: "South Wales Fire and Rescue have now confirmed the fire is completely extinguished and they will soon be leaving the scene.
"Structural engineers will now enter the westbound tunnel to carry out an assessment of any damage caused.
"Traffic congestion is now obviously a major concern and we would urge any motorists planning on using the M4 in south east Wales to make alternative arrangements.
"Whilst we are doing everything we can to inform motorists in real time, with live traffic updates and wide area signing, we would ask for your cooperation and understanding as we attempt to relieve pressures on the network and reopen the tunnels."
The westbound carriageway remains closed between J24 and J26.