The poll was the first to show that a majority of French people who are planning to vote — 51 per cent — are saying that they planned to vote “non”. The “yes” vote, meanwhile, has fallen to 49 per cent, with 53 per cent of those polled still likely to abstain.

It confirmed a drift that mainstream politicians have feared ever since President Chirac took the risky decision last summer to call a vote. In September last year the “yes” vote was 69 per cent, with the “no” vote struggling on 31 per cent.

The “no” campaign is riding on deep popular resentment towards M Chirac’s Government and a belief that now the EU is run by Britain and its allies.

A French “no” on May 29 would kill the constitution, making France the new bad boy of Europe and neutralising Britain’s own referendum. The treaty must be ratified by all 25 EU states. A “no” from a smaller country might be accommodated, but not from one of the Continent’s three big powers.

The opinion poll, carried out by the influential CSA organisation for le Parisien newspaper, delighted the “no” camp. Laurent Fabius, a former Socialist Prime Minister who has broken with his party line, said that it was “obvious that the French are getting an ever better understanding of what is in the constitution”.

Jean-Marie Le Pen, leader of the National Front party, said: “Maybe there is hope of saving France from the European poison.”

From the “yes” camp there was a whiff of panic. “It’s a real electric shock,” Gérard Larcher, the Labour Relations Minister, said. Jean-Pierre Raffarin, the Prime Minister, said the poll finding would be a blessing if it shocked the “yes” camp into a vigorous campaign.

Nicolas Sarkozy, leader of M Chirac’s Union for a Popular Majority (UMP) party, gave warning in his first campaign rally on Thursday night that “If France were to say no, Europe would be paralysed and France would be isolated”.

He promised: “I will deploy all my energy . . . I will convince French voters, one by one if need be. Choosing Europe is an historic choice.” He predicted a close fight, like the 1992 vote on the Maastricht treaty and monetary union, which was won by less than 1 per cent.

The constitution was drafted by a Frenchman, the former President Valéry Giscard d’Estaing, but support has melted away as the “no” camp has turned the referendum into a weapon to beat M Chirac’s unpopular administration and blast everything that France loathes in the 25-member EU.

Led by an unlikely alliance of left-wing Socialists and right-wing nationalists, the rejectionists have branded the constitution an Anglo-Saxon conspiracy designed to make the EU a pro-American, free-market jungle that will destroy the French way of life. The Right has gone farther, depicting it as an open door to Turkish entry to the EU.

“Europe has become the scapegoat for everything. Nobody is talking about the European treaty: they are talking about everything but the reality of the constitution,” Julien Dray, an MP and Socialist party spokesman, said.

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