A British No would destroy more than the treaty

If opinion polls are a fair guide, all European Union countries will ratify the new constitutional treaty - except Britain, which seems set to vote No in the referendum due in mid-2006. What would happen next is uncertain, but the EU would probably spend several years on inwardlooking institutional arguments. The Union would have less energy for dealing with security threats, economic challenges and its problematic neighbours.

In theory, a No from Britain would destroy the new treaty, but in practice the other members would not agree to keep working with the existing treaties. They believe the newly enlarged Union will not operate smoothly without the constitutional treaty. The other governments would not agree to renegotiate the new treaty, to make it more palatable to British tastes. They fear that if they re-open the treaty's hard-fought compromises they will end up with a less good deal.

Might the others ask the British to vote a second time, as the Danes and Irish did after rejecting the Maastricht and Nice treaties, in 1992 and 2001 respectively? Unlikely. Those treaties extended the EU's remit into new policy areas, allowing the Danes and the Irish to vote again with the addition of opt-outs from new policies and declarations to reassure voters on sensitive issues. But the constitutional treaty is mainly about changing EU institutions and voting rules. Only in justice and home affairs does the treaty give the EU significant new powers - but Britain already has an opt-out there.

Some governments would respond to a British No by trying to press ahead with the new treaty. The other 24 members could withdraw from the existing EU treaties, redraft the constitutional treaty among themselves, and then ratify it. Britain would have to negotiate an associate status similar to that of Norway or Switzerland, which enjoy access to the EU market but cannot vote on its rules. However, any scheme to exclude Britain would require the consent of all 24 other members - which is unlikely. Some of the countries that share Britain's Atlanticism and market-oriented approach to economics would not want to be left in an EU dominated by France and Germany.

But if France, Germany and their friends could not rescue the new treaty, they might try to set up a new organisation, with its own institutions and budget. However, the legal and political difficulties of establishing such a "hard core", and ensuring that it operated smoothly alongside the EU, would be immense. The fact that France and Germany would lead this core would make it inherently divisive. Many senior French politicians would support a hard core - Jacques Chirac has mused on the idea. But the scheme would not be viable without equally strong German backing, which is doubtful. Some of Germany's top diplomats would back a core, but not many politicians. The business community would oppose any scheme that led to closer ties to the French at the expense of the Anglo-Saxons.

If the countries that want a more united Europe failed to adopt the new treaty or set up a hard core, they would pursue other paths. First, they would try to salvage parts of the constitutional treaty. Most of it cannot be implemented without ratification, but some parts - such as the provision for a European diplomatic service - could be put into effect without breaching the current treaties.

Second, they would use the "enhanced co-operation" rules of the existing treaties to set up vanguard groups in particular areas, such as corporate taxation or research and development. Third, they would create other vanguard groups outside the framework of the EU treaties, in areas such as police co-operation and criminal justice, using the 20-year-old Schengen arrangements to scrap border controls as a precedent. Fourth, these countries would turn the eurogroup from an informal forum into a solid institution for the eurozone countries.

Thus within the broader EU, distinct but overlapping smaller groups would pursue closer cooperation. After a while, the countries in all these groups would emerge as the self-appointed leadership for the whole Union. This "messy core", dominated by France and Germany, would have one view of foreign policy, while the periphery, including many Atlanticist countries, would have another. A Europe thus divided would be hard-pressed to develop stronger foreign policies and effectively meet the many external challenges it faces.