The long and vigorous French debate on the proposed new constitution for the European Union has exposed one central truth that Tony Blair would prefer that this country’s voters did not grasp. Far from merely applying a bit of oil to the EU’s works, this treaty would fundamentally alter the political balance of Europe, greatly expanding the powers of the Union at the expense of EU member states.

Spain’s voters, those few who bothered to turn out, endorsed the constitution earlier this year almost absent-mindedly, easily persuaded by their political elite that a “yes” would safeguard the regional subsidies lavished on them since joining the EU, while earning them kudos for showing “European leadership”. Almost no one in France, by contrast, is so naive as to imagine that this vast, barely comprehensible “treaty of treaties” would, if adopted, preserve the status quo.

As for leadership, it is the loss of French influence in Europe that is widely feared. In both the “yes” and “no” camps, it is common ground that France will tomorrow be called to decide on a question of historic importance. Whichever way the French vote, the campaign itself will thus have rendered valuable service to all conscientious Europeans.

The French, who laid the foundations of what is now the European Union, designing and initially dominating the Brussels bureaucracy, understand better than anyone the intricacies of how the EU works. No country has been more deft at maximising for itself the advantages of EU membership, amassing EU farm subsidies while diluting or ignoring unpalatable regulations emanating from Brussels. No country has, until now, been more confident that the EU provided a continental arena for the more effective pursuit of its national ambitions.

It should compel attention throughout the EU that France sees this constitution as a momentous step – whether, as the “yes” campaign would have it, forging a “united Europe” capable of acting as a mighty counterpoint to the US in global affairs, or whether, as the treaty’s numerous opponents proclaim, reinforcing centralising trends in ways that threatened France’s sovereignty, identity and way of life.

The French debate has also demonstrated how this bewildering treaty — some 200 pages in the French edition — signally fails to provide the clarity and certainty about functions and powers that EU leaders promised in their Laeken Declaration when they set up the drafting convention. In crucial segments, its ambiguities permit of multiple interpretations.

Thus the “no” camp in France, an improbable coming-together of far Right, far Left, and dissident Socialists and Gaullists, claims that the treaty would empower the EU to impose “Anglo-Saxon capitalism” on France, tearing into the country’s thick webs of social and labour market protection. These are not real concerns about Europe’s direction, but an articulation of national anomie.

Opposition in Britain, by contrast, is influenced by concern that the constitution would have precisely the opposite effect, centrally imposing an ossified and perilously uncompetitive “European social model”.

No doubt the French debate is strongly influenced by popular attachment to an interventionist state, by acute anxieties about economic stagnation and chronically high unemployment — and by a desire to punish President Chirac. But it is extraordinary that even some of those intimately involved in drafting the confused and confusing constitution — such as Lord Kerr of Kinlochard — concede that it “got out of control” and the EU’s current predicament is “a mess”.

A French “non”, while generally expected, is not a foregone conclusion. With both mainstream parties officially in favour, although the Socialists are split, with almost every newspaper supporting a “yes” and with the “yes” camp dominating airtime in a campaign into which the State has poured more than €400 million, a “no” would be a rebuke to almost the entire political establishment.

Still, when President Chirac announced last year that the treaty would be put to a referendum, rejection appeared unthinkable.

The failure, in France as in Britain, of the “yes” campaign to present a coherent and convincing case for the constitution is partly responsible for changing voter intentions. M Chirac claims that the constitution will defend the French “social model” — and that if France votes “no”, it will “cease to exist politically in the EU”. That line is flatly contradicted by Nicolas Sarkozy, the Centre-Right’s most popular leader, who urges a “yes” vote to force change on France, arguing that “the best social model is one that provides jobs for everyone. In other words, it is no longer ours.”

Valéry Giscard d’Estaing, the constitution’s architect, loftily advises voters not to waste time dwelling on the voluminous small print, reposing their trust in the high-minded stuff about freedom and human rights contained in its short preamble. That, above all, is advice that every thinking voter should resist.

It is that very same small print that has convinced this newspaper, while it shares M Sarkozy’s reformist instincts and wants the EU to work better, that this constitution is the wrong way to set about reforming and reinvigorating the EU. The constitution’s opacities would render the EU even more incomprehensible to its citizens; and its ambiguities favour further empire-building by Brussels; and it can lay no claim to responding sensibly and swiftly to enlargement, since the reforms it proposes to EU decision-making would not take effect until 2009.

This has been a national debate, as it is a national choice. The verdict is for the French people alone. But it is as Europeans that we hope that France, a founding mother of the EU, votes “no” — and thus opens the way to a genuine reform that would make the Union the servant, rather than the unelected and barely accountable master, in the European house of many proud nations that we wish to see strengthened and equipped for a changing world.