Part of the Heads of the Valleys Road is shut following a crash near Abergavenny.
The road is shut in both directions from the A465 Skenfrith junction to A40 Monmouth Road following the crash.
Traffic is queueing following the incident on the busy road. Traffic monitors report that congestion is on routes through Abergavenny as drivers avoid the scene.
Gwent Police have advised drivers to find an "alternative route for your journey".
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