Members of the CGT union are taking part in a 72 hour walk-out called Opération ports morts. It will affect seven ports until at least Friday.
The ports affected are Le Havre, La Rochelle and Marseille, which are completely blocked. Other ports affected are Calais, Dunkirk, Rouen, and Nantes-Saint-Nazaire.
Brittany Ferries have announced cancellations between Portsmouth and Le Harve, and ferries between Marseille and Corsica have been cancelled until Friday by the Corsica Linea ferry company.
Some ferries, although not all, are sailing to and from Dunkirk and Calais.