Ferries between Dover and Calais will be disrupted by a national strike in France on Thursday.
P&O Ferries said services to and from the French port will be suspended from 07:00 GMT for nine hours.
Dover will still be open with Dunkerque services running as normal but travellers are being urged to allow extra time for journeys.
A day of strike action across France is being staged to protest about plans to raise the official pension age.
The national stoppages will involve transport, school and health workers.
P&O said it could not anticipate the extent of disruption to services, adding that “it is possible that our wider operations could be affected during the day”.
When ferries resume after strike action ends at 16:00 GMT, there will be a shuttle service between Dover and Calais until all traffic is cleared.
Danish ferry company DFDS has told customers due to travel from Dover to Calais on Thursday that alternative arrangements would be made via Dunkerque.