Three vehicles have crashed on the A27 by the Ashcombe roundabout at Kingston just outside Lewes.
The accident, involving a dustcart and two other vehicles, has left commuters in a five-mile queue back to the stretch of the Brighton bypass between Patcham and Hollingbury.
The Highways Agency said: “There are currently delays of 20 minutes.”
The agency said that conditions should be back to normal by between 9am and 9.15am.
Commuters heading east from Brighton by train – and passengers trying to reach the Channel Tunnel at Ashford – continue to face delays after a land slip near Glynde.
Speed restrictions on the line and changes to the timetable are expected all week.