It would be betraying no secrets to say that the French president, Jacques Chirac - who announced the date of the keenly-awaited plebiscite late last week - would have given a great deal not to have had to do so. He is well aware that, if it were all to go pear-shaped, the first head on the block would be his.

Had he had any option other than calling a referendum he is by no means certain of winning, Mr Chirac would have taken it. But under pressure from all France's political parties, from both the pro and anti-constitution camps, and from the French electorate, he knew he could not be seen to be dodging the issue by relying on a straightforward vote in parliament.

So it had to be a referendum - and it presents Mr Chirac with a very big problem indeed. The result is, to say the least, uncertain. France has a history of using plebiscites to punish the government of the day - and the government of the day is very unpopular indeed.

Moreover, France - while pretending in public to be a nation of fervent and deeply committed Euro-keenies - is, in reality, fundamentally conservative and essentially eurosceptic. As Laurent Joffrin, a leading leftwing commentator, said recently: "Let's be frank - the first instinct of the French is to vote no."

All the polls show the pattern of France's referendum on the 1992 Maastricht treaty inexorably repeating itself. Starting with broad public support (65% only a few months ago), the yes camp finds itself becoming increasingly unpopular (now below 50%, according to some surveys). In 1992, it ended up carrying the day by a whisker, but this time it may not.

Right now, the no camp - composed of an unholy and disparate alliance of the National Front, the Communists, the Trotskyists, the left-leaning republicans, the right-leaning sovereignists and a significant minority of the Socialists - is busy earning points. The yes campaign, comprising most of the ruling UMP party, its centrist UDF allies and a majority of the Socialist opposition - has yet to get off the ground.

So worried is Danny Cohn-Bendit, the former student revolutionary turned emblematic pro-European Green, that he has proposed organising joint yes campaign rallies alongside rightwing politicians from the UDF and even the UMP, to whom he would not normally give the time of day.

Mr Chirac has every reason to be concerned. If the no vote won the day, he would go down in history as having fallen at the one big hurdle of his presidency. Having invested himself so heavily in the yes campaign, he would be a lame duck president for the last two years of his second term (elections are due in 2007), and could kiss goodbye to any realistic hope of standing for a third.

But the consequences of a victory for the no campaign in France do not stop there: the first thing the president would do in the event of the electorate rejecting the treaty would be to attempt to salvage something from the wreckage and give his government a new impetus by replacing the unfortunate PM, Jean-Pierre Raffarin.

The position of Mr Chirac's arch-rival, the young and thrusting Nicolas Sarkozy, would, inevitably, also be weakened, although to a lesser extent - as the head of the UMP, he has been obliged to throw himself behind the yes campaign. However, unlike Mr Chirac, he was not one of the treaty's instigators, and nor was it he who signed it in Rome on behalf of the nation. Its defeat would not be entirely his defeat.

On the left, the fallout from a no victory would be equally dramatic. Having wholly unexpectedly piloted his party through a highly successful 2004, with sweeping victories in regional, local and European elections, the witty but owl-like Socialist leader, François Hollande, would see his growing authority fly out the window.

He faces just as difficult a coming three months as Mr Chirac: his number two, the former prime minister Laurent Fabius, is just one of a fistful of leading Socialists threatening to ignore the result of the party's internal referendum last year and campaign, more or less actively, for a no vote. Many of France's more traditionally-minded Socialists see the constitution as Thatcherite blueprint for a union that ignores France's emphasis on worker protection, public services and welfare guarantees.

A French no vote, then, could hasten the demise of Mr Chirac, the man who has dominated the French right for the past 30-odd years; could seriously taint the prospects of his main rival, Nicolas Sarkozy; and could bring to a crashing halt the hitherto unstoppable rise of Mr Hollande, at present the only real prospect the Socialists have for a convincing presidential candidate in 2007.