Sussex Roads Police announced on Twitter at 3.25pm that they were attending the scene of the collision.
They said that traffic had been diverted via the Bolney off-slip and back on again from the roundabout.
Sussex Roads Police have now added that the southbound carriageway is closed from the junction with the A272.
“Motorists are asked to please avoid the area and find alternative routes where possible,” they said.
AA Traffic News is also reporting that traffic is moving slowly.
We will update this story as more information becomes available.