The clear French No vote to the European Union's constitutional treaty is a grave blow both to President Jacques Chirac and to the EU as a whole. Not for the first time, the French president has made a huge political miscalculation, by calling a referendum at a moment of deep unpopularity for his own government, and then by failing to fight it in a whole-hearted and positive way. But the price will be paid not only by the French government. It will also cause confusion and possibly a political stalemate in the wider EU.

The reasons for the French No are diffuse. There is no simple answer to the voters' revolt. Part is clearly a backlash against Mr Chirac from the left, combining frustration at the failure of his government to reduce unemployment, and a powerful anti-globalisation movement blaming international competition for France's plight. But that fear of competition has been confused with anxiety about the effect of EU enlargement, with France losing influence in a 25-member union and losing jobs to cheaper labour in the east. On the right, there is strong opposition to launching membership talks with Turkey later this year.

There was never an instant Plan B on how to respond to the French decision. All 25 members have signed the treaty, and all are committed to debating and ratifying it by November 2006. Several more may say No, including the Netherlands where a referendum is to be held on Wednesday. But that is not a good reason to stop the process. Nine states have already said Yes, including two of the largest, Germany and Spain. Their views should not simply be dismissed because France has voted No. Nor should those of the countries yet to decide.

What the debates in France and the Netherlands have demonstrated is a great desire among ordinary voters to have a real say on the future of the EU. They have not been properly consulted for far too long. The wrong reaction would be for EU leaders to retreat once more behind closed doors, call off the political process and try to save the parts of the treaty they like best in a constitutional fudge.

They must avoid the other danger lurking in the French No vote: that Mr Chirac will seek to revive his political popularity in ever more defensive positions, encouraging others to do exactly the same. That is the way towards prolonged political stalemate on economic reform, the Doha round of trade liberalisation, negotiations on the long-term EU budget, and the future enlargement process. Hence the need not to over- react and to seek more time for a level-headed debate.

On balance, it would seem most sensible to allow the ratification process to continue, even if it does mean that others may vote No, including the UK. Only when it is clear where all the members stand can a sensible effort be made to rework the treaty. The French No is a shock. It is also an opportunity: for a fundamental debate to be held on the shape of the future EU.