UPDATE 2.12pm: All lanes are now open and traffic is rreturning to normal, though congestion remains on the stretch between junctions 25 (Caerleon) and 23 (Magor).

A LANE of the M4 westbound at junction 25 (Caerleon) is blocked, and traffic is slow due to accident.

Traffic is currently queuing back some four miles beyond junction 24 (Coldra).