UPDATE: The road has been reopened.
They said traffic is now being diverted and there will be further updates when the road is clear.
AA Traffic News has reported severe delays of 18 minutes on the M23 southbound between junction 10A (B2036 Balcombe Road, Crawley South/Balcombe) and junction 11 (A23, Pease Pottage).
The AA said the average speed is five mph.
More updates as we get them.