The M4 Prince of Wales Bridge will be closed for several hours following a crash in which a lorry left the carriageway.
All lanes were shut in both directions from J22 to J23 as an air ambulance attends the "very serious" incident this morning (September 4).
Motorists have been advised to use the M48 bridge as a diversion and traffic has been left trapped on the westbound carriageway.
Highways England has said the accident involved a lorry which left the carriageway and went down the embankment.
Get the latest updates in our M4 Severn Bridge crash live blog.
A diversion route has been put in place to avoid the crash site.
A spokesperson for the agency said: "The M4 Prince of Wales Bridge has been closed in both directions due to a serious incident.
"A HGV has left the carriageway and gone down the embankment.
"Emergency services are currently at the scene and the closure is expected to remain in place for several hours.
"Road users are being diverted via the M48 (Severn Crossing)."
What police have said
Gwent Police has warned motorists to avoid the area due to heavy congestion.
A police spokesperson said: "Gwent Police is currently responding to a report of a lorry that has driven down the embankment and onto land alongside the M4 Westbound at the end of the Prince of Wales Bridge.
"Officers are currently on the scene and the M4 westbound road is closed.
"We are advising motorists to avoid the area due to heavy congestion.
"Anyone with any information or anyone who may have witnessed the incident or has dash cam footage is urged to contact Gwent Police on 101 or Direct Message us on Facebook or Twitter, quoting log 129 04/09/19
"We will update you as soon as we have more information."
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