An accident in the Brynglas Tunnels in the early hours of this morning means the M4 Eastbound is closed near Newport.
The motorway's shut between Junctions 28 and 25A; Tredegar Park and the Grove Park Roundabout.
Traffic is queueing for around four miles back to J30 the A4232 at Cardiff Gate and on the A48(M) to J29a at St Mellons.
There's a diversion on the Southern Distributor Road where traffic's very heavy.
Police say they hope to re-open the motorway by 9am.