France's top administrative body has advised the government that any total ban on face-covering Islamic veils could be unconstitutional.
The State Council also said a ban could be justified in some public places.
Prime Minster Francois Fillon had asked the council for a legal opinion before drawing up a law on the subject.
However, an MP from President Nicolas Sarkozy's party was quoted as saying that those drafting the legislation might ignore Tuesday's ruling.
In the ruling, the council said any law could be in violation of the French constitution as well as the European Convention for the Protection of Human Rights and Fundamental Freedoms.