A Welsh Government spokesperson said, “South Wales Fire and Rescue have now confirmed that 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.”