The National Assembly passed the Civil Solidarity Pact (PACS) by 315 votes to 249.

The BBC's James Coomarasamy: "Gay groups counter that the legislation doesn't go far enough"

Conservative opponents immediately said they would ask the Constitutional Council to rule whether the law was unconstitutional.

The PACS allows unmarried couples to register their union and enjoy some of the tax, legal and social welfare benefits associated with marriage.

It is intended to allow gay and heterosexual couples who are not married to "organise their common life".

Partners who want to separate will be able to do so via a letter of separation.

Religious leaders have strongly denounced the law, saying it enables a form of homosexual marriage.

'Bitterly contested'

The PACS was introduced by the ruling socialists and the government's majority made approval virtually certain.

It has been one of the most bitterly-contested pieces of social legislation for years, opposed by conservatives and by leaders of the Christian, Jewish and Muslim faiths in France.

Opponents fear it will eventually lead to gay couples being able to adopt children, although the government has opposed any such move.

Five million people

According to Justice Minister Elisabeth Guigou, the bill will improve the lives of more than five million people.

Last November, parliament rejected a conservative bid to sink the controversial bill.