But the plan faces stiff opposition from the Roman Catholic Church and social conservatives, with huge numbers turning out for protest marches in Paris in recent weeks.

Another big rally against the law is planned for next month.

Cardinal Andre Vingt-Trois, the archbishop of Paris, voiced his opposition at a meeting of French bishops in Lourdes last year.

Opening up marriage to same-sex couples "would be a transformation of marriage that would affect everyone," he said. At the same time, failing to recognize gender difference within marriage and the family would be a "deceit" that would rock the foundations of society and lead to discrimination between children, he said.

Other religious groups, including Muslims, Jews and Buddhists, have also expressed their concern over the draft bill, and many lawmakers and hundreds of mayors are against the legislation.

The legislation has won wide backing from gay rights advocates, however.

The French gay, lesbian and transgender rights group Inter-LGBT has said the law, if passed, "would be a major advance for our country in terms of equality of rights."

Lawmakers have a "unique opportunity" to put an end to outdated discrimination, the group said in a statement. "The law must allow all couples to unite themselves as they wish and must protect all families, without discrimination on the grounds of sexual orientation or gender identity," it said.

A law legalizing civil unions was introduced in 1999 in France under a previous Socialist government.

Known in France as the PACS (pacte civil de solidarite), the civil union agreement can be entered into by gay or straight couples and confers many but not all of the rights of marriage.