The A350 is blocked near Lacock, at the Folly Lane junction, after a two vehicle accident involving a black Renault Clio and a blue Ford KA.
Police, who were called to the scene at 3.47pm, are advising motorists to avoid the area as traffic is queuing in both directions.
They believe the road will be closed for another hour at least.
An ambulance is on the scene as paramedics treat a 35-year-old woman, believed to be the driver of the Ford KA, for minor injuries.
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