The vehicle has come to a halt on the westbound carriageway in the outside lane.
There are also reports of an accident on the same stretch of road, involving two vehicles.
It is not known whether the two incidents are related, but the accident was reported at around 6.10pm and recovery is under way.
Traffic heading towards Dorset is slow as a result, with queues as far back as the M271, and traffic heading towards Southampton is also slow, due to people looking at the scene.
The latest information suggests that it could take until around 10pm to clear the scene, due to difficulties moving one of the cars.