One of two slicks 20m wide and 150m long was spotted at the mouth of the Bassin d'Arcachon and had entered the inlet.
In response, French President Jacques Chirac has angrily criticised "hooligan" businessmen responsible for oil spills such as the Prestige disaster now menacing France's Atlantic coast.
He said France and Europe would not allow "corrupt businessmen" to continue exploiting the weaknesses of the current system of checking oil tankers.