The wearing of full veils has been the subject of a fierce debate in France
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.
"It appears to the State Council that a general and absolute ban on the full veil as such can have no incontestable judicial basis," it said.
'Clear message needed'
But it added that rules requiring faces to be uncovered in public places such as schools, hospitals and law courts could be justified for security reasons, to combat fraud and to meet the needs of some public services.
The State Council is required to give an opinion before any major piece of legislation is drafted in France.
However, Jean Leonetti, the deputy parliamentary leader of Mr Sarkozy's UMP party, said a ban "needs to be complete or else it is misunderstood".
"We still are of the view that a message needs to be sent that is clear and does not waver in terms of its implementation," he said.
President Sarkozy has said more than once that the face-covering veil is not welcome in France, and that he wants a law restricting it.
In January, a French parliamentary committee recommended a partial ban on Islamic face veils that could be imposed in hospitals, schools, government offices and on public transport.
There are several types of headscarves and veils for Muslim women - those that cover the face being the niqab and the burka. In France, the niqab is the version most commonly worn.
The interior ministry says only 1,900 women wear full veils in France, home to Europe's biggest Muslim minority.