Employees of a ferry service recently sold by Eurotunnel renewed a blockade of the northern French port of Calais on Monday after a court rejected their bid to extend the service's charter contract with Eurotunnel.
Access to the boats on the docks was not possible in the afternoon, said union and port sources.
Strike action by around 400 workers last week led to major traffic jams of lorries, prompting migrants around the port to try and stowaway on trucks bound for Britain.
Eurotunnel, the operator of the undersea rail link between England and France, in June agreed to sell its Calais-to-Dover ferry business to Denmark's DFDS to end a lengthy battle with British competition authorities.
SCOP Sea France, the co-operative of workers that runs the ferries, asked a commercial court in Boulogne-sur-Mer to extend its contract with Eurotunnel and prevent it from being dissolved after the sale. The court on Monday rejected that request.
DFDS, which is set to take over operation of the ferries on July 2, has pledged to keep 202 out of 577 workers, a level the union sees as unacceptable.
The ferry workers had initially tried to buy the business of operating the two ferries from Eurotunnel themselves but failed.
(Reporting by Pierre Savary; Writing by Leila Abboud; Editing by Mark John)