The Second Severn Crossing is likely to be closed for the rest of the day after a lorry crashed off the bridge and down an embankment.
The male driver of the lorry has been airlifted to hospital in Cardiff with serious injuries after the Wednesday morning crash.
Emergency services were called to the scene at 10.30am and remain there.
It was reported that a lorry had left the carriageway and had gone down the embankment and onto land alongside the M4 westbound at the end of the bridge.
The bridge has been closed in both directions as police remain at the scene and structural engineers continue to inspect the damage caused and carry out emergency repairs.
Chief Inspector of operational support, Martyn Smith, said: “Currently, the bridge is closed east and westbound while engineers assess the damage to the barriers and carry out emergency repair works.
"We anticipate the bridge will be closed by the Highways Agency for the remainder of the day. Highways England have confirmed that the westbound carriageway is expected to remain closed between Junction 21 and Junction 23 until at least 6am tomorrow morning. We are advising motorists to make alternative routes today."
There are long tailbacks on the M4 on the approach to the bridge and on the M48 Severn Bridge as motorists seek out an alternative route.
Traffic cameras show the length of delays on the motorway
Roadworks on the original Severn Crossing are not helping the congestion.
Cardiff Airport has also warned passengers to leave plenty of time getting to the airport because of the delays.
Officers are now investigating the circumstances of the incident and are asking anyone with any information or has dash cam footage to call Gwent Police on 101 or message them on Facebook or Twitter, quoting log number 129 04/09/19.
