One person has been taken to hospital following the crash, which involved a Toyota IQ2 and a lorry.
Emergency services were called to the scene at 8.34am.
A spokeswoman for the Welsh Ambulance Service said: "We sent two rapid response cars and an emergency ambulance to the scene, and one person was taken to the Royal Gwent Hospital."
All lanes have since reopened.
A LANE is closed on the M4 westbound following a crash.
Delays of up to 15 minutes are being reported stretching back from the scene of the incident at Junction 27 (High Cross) and beyond the Brynglas Tunnels.
An eyewitness said the third lane is the one that is closed, with Gwent Police and Welsh Ambulance service crews at the scene.