MyFerryLink workers on Monday blocked the northern French port of Calais for the second time in a week, angered at plans to sell off ferries to a rival firm, a union leader said.
"Right now the action is concentrated on the port," said Eric Vercoutre, head of the main maritime union organising the industrial action, without ruling out wider action in the days ahead.
Last Tuesday hundreds of protesting French sailors blocked the Calais port and forced the suspension of Eurotunnel services between France and Britain in both directions after torching tyres on the tracks, causing traffic and travel chaos.
That demonstration also sparked a scramble among some of the thousands of illegal migrants camped in Calais hoping to reach Britain, as they battled to hitch a ride on vehicles stuck in traffic.
The French ferry company workers are protesting plans by Eurotunnel to sell two of their cross-channel ferries to rival Danish firm DFDS.
Eurotunnel announced in May it was halting its operational partnership with MyFerryLink due to legal complications, and this month decided to sell its ferries to DFDS.
The Danish ferry operator has said it plans to provide work for only 202 of the current 600 MyFerryLink workers.
The sale of the two ferries would leave MyFerryLink with just one boat in operation, meaning over half the workforce faces redundancy.