Operation Stack has been activated in Kent in response to a build-up of lorries heading to France caused by industrial action in Calais.
Freight traffic is being held on the coastbound carriageway of the M20 between Junctions 8 and 9.
The “emergency measure” is designed to prevent gridlock on Kent’s road network.
Ferries between Dover and Calais are suspended due to a 24-hour strike by staff at the French port, which began at 8am on Thursday.
Ferry operators DFDS and P&O Ferries normally run a total of up to 50 sailings per day on the route.
DFDS has switched one of its Calais ships to the Dover-Dunkirk route to provide extra capacity.
Freight vehicles and passengers due to travel between Dover and Calais with DFDS on Thursday were shipped onto Dover-Dunkirk sailings instead.
The Port of Dover said in a statement: “We will continue to monitor the situation closely in liaison with our ferry partners and the Port of Calais in order to resume normal operations as soon as possible.
“Please note that there may be delays to services when the Port of Calais re-opens.
“We apologise for any delays or inconvenience as a result of the industrial action in France.”