This time there really is no Plan B to deal with a decisive rejection from the very heart of the European Union, writes Denis Staunton in Brussels
It will, if you believe the anxious policy-makers and commentators of Brussels, provoke Europe's biggest political crisis for half a century, a catastrophe for European integration that could paralyse the European Union for years.
Almost everyone at the heart of the European project agrees that a French No vote would be a disaster, both for the constitution and for the EU itself, but nobody has a clear idea of what to do about it. This time, there really is no Plan B and Europe's leaders are preparing to enter uncharted territory.
For the past year or so, officials in some European capitals have been working quietly on contingency plans in the event of one or more countries rejecting the constitution. The problem is that all these plans were based on the assumption that France and Germany would be among those supporting the constitution, with Britain and one or two small states voting against.
In such circumstances, reluctant member states could be asked to vote a second time, perhaps - as in Denmark after Maastricht and in Ireland after Nice - following the agreement of special protocols or declarations to address particular concerns.
Failing that, those who reject the constitution could be persuaded to accept a form of associate EU membership, allowing the majority to press ahead with the constitution.
Officials in Paris and Berlin have considered options for a "core Europe" around France and Germany, with a pioneer group of countries pursuing closer integration and perhaps voting in the Council of Ministers as a bloc.
None of these solutions would be of use, however, if France votes No on May 29th, particularly if, as opinion polls currently indicate, the Netherlands follows suit three days later.
According to the constitution, if at least four-fifths of the member states approve it and the rest do not, "the matter will be referred to the European Council" of EU leaders.
A new constitutional arrangement for Europe without France is unthinkable, however, because of that country's size, its status as a founding member state and its unrivalled influence on the process of European integration.
Abandoning the constitution is problematic, too, not only on account of the inadequacy of the Nice Treaty but because it would signal that the European project had no political direction.
Finding the right response to a French No is all the more difficult because those who reject the constitution are likely to do so for such diverse reasons. Traditional anti-Europeans from the extremes of right and left have been joined by a growing number of French citizens who believe that the constitution offers, not too much, but too little Europe.
This second group wants a constitution that promises more action at a European level to protect minimum standards for workers, for the environment and for social standards. They are unhappy with the liberal economic policies that José Manuel Barroso has put at the centre of his EU Commission's programme for the next five years.
The Financial Times Deutschland reported on Thursday that some EU governments have already written off the French referendum and are trying to co-ordinate their response to a No vote. The paper said that Luxembourg's EU presidency was determined to proceed with ratification of the constitution regardless of the French vote and would go ahead with the Grand Duchy's own referendum on July 10th.
According to an analysis for the Federal Trust (www.fedtrust.co.uk) by Jo Shaw, Professor of European Institutions at the University of Edinburgh, any other course would be illegal.
"Having agreed and signed the constitutional treaty, the member states are under an obligation under international law to seek ratification of the treaty at the domestic level.
"Their obligations to their fellow high contracting parties under international law - not to mention their obligations under Article 10 of the existing EC Treaty to show 'loyalty' to the EU as it stands - require them not to pretend simply that the constitutional treaty does not really exist," she says.
After Luxembourg in July, the next scheduled referendum is in Denmark on September 27th. Until now, opinion polls have suggested that the Danes will vote Yes, but a French No could boost the Danish anti-constitution campaign.
The EU presidency hopes that proceeding with the ratification process will help to limit the destabilising effect of a French No vote on EU politics. But before long, the EU will have to face the possibility that the constitution cannot come into force in its present form.
Some EU officials are looking at ways of introducing some of the changes envisaged in the constitution without altering the treaties. Others suggest that Part III of the constitution, which deals with EU policies, could be abandoned, leaving only Parts I and II - a general description of the Union's values, objectives and relations with the member states and the charter of fundamental rights. In this scenario, elements of Part III could be incorporated into the Nice Treaty without the need for ratification.
One obvious problem with this plan is that voters who rejected the entire constitution could be reluctant to vote for Parts I and II in the knowledge that much of Part III will be introduced without asking them.
None of these technical remedies would address the political crisis triggered by a French No, and there is little sign in Europe of a political leadership up to the task.
A referendum defeat would leave Jacques Chirac too badly weakened to lead any political initiative for Europe, and a leadership alliance between Tony Blair and Germany's Gerhard Schröder is a remote prospect.
As the body dedicated to upholding the general interest of the EU, the European Commission is the natural candidate to assume the leadership role at such a time of crisis. Unfortunately for the European project and those who wish it well, Mr Barroso and his undistinguished commission appear to be more part of Europe's problem than its solution.