A lorry carrying a refrigeration trailer filled with milk was involved in an accident which saw both milk and diesel spilled onto both carriageways just after midnight.
The road has been closed in both directions; northbound from Ashington B2133 to the Partridge Green junction with the B2135, and southbound between the Buck Barn crossroads and Dial Post.
West Sussex Highways said the diesel spillage covered more than a mile-and-a-half on both carriageways and crews were working to get the road open.
The trailer became detached as the vehicle was heading north just past the junction with Dial Post. It overturned on the central reservation and blocked part of a lane.
Highways said during the crash the lorry’s fuel tank was damaged and the vehicle continued for about half-a-mile leaking diesel.
Fuel was also spilled on the southbound carriageway for just under a mile as well as both areas where the lorry turned around as it travelled back. A manhole cover was also damaged in the incident.
Police said no-one was injured and no arrests had been made.
A West Sussex Highways spokesman said: “We are doing all we can to make the road safe and get it re-opened as quickly as possible.
“We realise this has caused a major disruption for road users and would like to thank them for their patience and understanding.”
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