The French opposition Socialist Party has voted to support the "yes" campaign in next year's national referendum on the European Constitution.
Francois Hollande and Laurent Fabius took opposing views
Party leader Francois Hollande said 59% of members had approved the document in an internal vote.
The decision will be welcomed by many European governments which feared the Socialist Party could persuade French voters to reject the constitution.
All 25 EU members must ratify the document for it to come into force.
The result will come as a relief not just to the Socialist leadership, but also to President Jacques Chirac who had offered the French people the referendum, says the BBC's Caroline Wyatt in Paris.
Around half the French electorate say they are undecided about which way to vote in the national referendum.
This result means the constitution will now be backed by both the left and the right, making it more likely that the French will vote "yes", our correspondent says.
Mr Hollande said his party's debate on the constitution had been intense, but that he believed it would help push forward the political organisation of Europe.
The former socialist prime minister, Laurent Fabius, who is Mr Hollande's deputy and had campaigned for a "no" vote, said on Thursday night he would not step down from his party post.
"I took my risks... but we will all be loyal" to the majority's choice, he said.
He had argued that the constitution was too Anglo-Saxon and pro-market - more about creating a free trade area than a strong Europe based on the French social model.
Turnout was high with some 80% of the party's 120,000 members voting.
This means the Socialists will now campaign in favour of a "yes" vote nationwide next year.