A divided France was set to vote Sunday on the European Union's first-ever constitution in a high-stakes referendum crucial to future EU integration and the political legacy of President Jacques Chirac.

After a campaign that mesmerized and polarized the country, final opinion polls suggested that opponents of the landmark EU charter would prevail, a decision that would effectively stop the treaty dead in its tracks.

Rejection of the constitution, which aims to streamline decision-making in the expanded 25-member bloc, would likely spark a period of political crisis both across the continent and in France, one of the EU's six founding members.

The country's 42 million eligible voters were to cast their ballots at polling stations from 8:00 am (0600 GMT) until 8:00 pm, with residents of Paris and Lyon given an extra two hours to cast their ballots.

No results were to be released until 10:00 pm (2000 GMT), so as not to prejudice last-minute voters. Due to the time difference, the 1.4 million voters in France's overseas territories went to the polls on Saturday.

When Chirac, whose ruling center-right Union for a Popular Movementis leading the "yes" campaign, called for a referendum on the EU text last year, supporters of the treaty had a comfortable lead in the opinion polls.

Now, the "no" camp -- capitalizing on the increasing unpopularity of the government -- is ahead with between 51 and 56 percent of the vote, and the 72-year-old French leader is counting on undecided voters to turn the tables.

Chirac's UMP is campaigning alongside its junior partner in government, the Union for French Democracy (UDF), as well as the opposition Socialist party (PS) and the Greens -- a coalition of France's mainstream political elite.

They are facing a disparate "no" camp made up of the far-right National Front of Jean-Marie Le Pen, the Communist and Trotskyist parties, nationalist Euroskeptics and the former Socialist prime minister Laurent Fabius.

Pollsters say that pro-European Socialist party supporters -- tempted to follow Fabius and vote "no" but fearful of the consequences -- will be the key, along with some one in five French voters who will decide at the last minute.

Chirac, who has staked his prestige on approval of the treaty, has warned voters that a French "non" would diminish the country's influence in Europe, indeed making it the "black sheep" of the continent.

Rejection of the treaty would have a profound political impact in France, tarnishing Chirac's place in the history books, prompting a government reshuffle and deepening the rift in the Socialist party.

No matter what the outcome on Sunday, Chirac is expected to dismiss his unpopular Prime Minister Jean-Pierre Raffarin, with Interior Minister Dominique de Villepin and Defense Minister Michele Alliot-Marie his possible successors.

Should France reject the treaty, Chirac -- who celebrated his 10th year as president earlier this month -- would be unlikely to seek a third term in office in 2007, opening the door to his arch-rival, UMP leader Nicolas Sarkozy.

Chirac has however ruled out his resignation should the "no" camp prevail.

Other EU countries fear that a French "non" also would unleash a domino effect across the continent, influencing voters in the Netherlands, Poland, Denmark and Britain to say "no" in future referendums.

A string of European leaders travelled to France to make the case for the constitution including German Chancellor Gerhard Schroeder and Spanish Prime Minister Jose Luis Rodriguez Zapatero, who wrapped up the campaign on Friday.

The constitution must be ratified by all 25 member states. So far, nine EU countries have approved the treaty -- Austria, Germany, Greece, Hungary, Italy, Lithuania, Slovakia, Slovenia and Spain.