A water tanker has rolled onto its side on the M4 Motorway in Greystanes, blocking one eastbound lane.
Traffic is queued about four kilometres from the accident scene.
All westbound lanes of the motorway remain open however traffic is also heavy as motorists slow to look at the accident.
Motorists travelling in either direction are advised to allow plenty of additional travel time and to expect significant delays.
Emergency services and traffic crews are at the accident site working to clear it as quickly as possible.