The French National Assembly has approved the most important article of a bill to legalise same-sex marriage, reports BBC News.
Deputies voted 249-97 in favour of redefining marriage as being an agreement between two people - not just between a man and a woman.
President Francois Hollande's Socialists and their left-wing supporters backed it, opposed by many opposition UMP and centrist MPs.
The proposals have generated protests and counter-protests for months.
Opinion polls suggest that around 55-60% of French people support gay marriage, though only about 50% approve of gay adoption.
Correspondents say the ease with which the article passed suggests the bill as a whole will pass.
Debates are expected to go on for more than a week, as MPs discuss hundreds of amendments, most of them filed by the centre-right opposition.
On the way they are expected to approve the other key measure in the bill, which would allow gay couples to adopt children.
The bill marks one of France's biggest social reforms since the abolition of the death penalty in 1981.
"We are happy and proud to have taken this first step," Justice Minister Christiane Taubira said.
"We are going to establish the freedom for everyone to choose his or her partner for a future together."
Read more of this report from BBC News.