National Highways said: "At around 02:00 Wiltshire Police responded to a serious single vehicle collision involving a car which left the eastbound carriageway near Leigh Delamere services.
"The carriageway was closed to allow Wiltshire Police, Dorset and Wiltshire Fire and Rescue Service and South West Ambulance Service to work at the scene of the collision.
"National Highways Traffic Officers attended the scene and assisted the emergency services by providing traffic management at both the scene and at J18.
"All traffic caught within the closure was turned and released via rearward relief.
"Although no definite timeframe can be given for re-opening at this stage, early indication suggest the closure will remain in place throughout this morning and into the afternoon."
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