Eurosceptics celebrate a triumph of the little people against the Euro juggernaut. Ireland's 'no' vote against the treaty on the European constitution is, in such minds, the brave assertion of democracy against bureaucracy. The European elite in Brussels, with its dark plans to hobble Europeans everywhere, deserves a good kicking for producing an unloved, incomprehensible set of reforms. It has got it. Ireland has stood up for Europe.

This is nonsense from top to bottom, a farrago of lies and disinformation. The European Union is a painfully constructed and fragile skein of compromises that allows 27 democratic states on our shared continent to come together and drive forward areas of common interest to further their citizens' well-being. The elite that plots this is a nonexistent phantom invented by populist demagogues. The beleaguered, unloved treaty would have improved Europe's effectiveness and tried to address its much talked about democratic weaknesses.

The reality is that Ireland's 'no' voters have trashed an EU that is precious but weak. Most 'no' voters, grabbing on to the worst fear rather than reasoned fact, have unknowingly set in train a political dynamic that, unless carefully handled, could lead not just to Ireland but Britain leaving the EU. Everybody will be the poorer.

Sometimes, fatalistically, I think this may have to happen. Eurosceptics, such as Ireland's leading 'no' vote campaigner Declan Ganley, like to position their fierce and unjustified attacks on the actual Europe we have as being pro-European because today's EU does not correspond to some impossible notion of Europe that meets their own very particular prejudices. Such is the flaw of referendums as a means to practise reasoned democratic decision-making that the only way voters will come to realise that the sceptics are wrong is to be forced to live through the consequences of their vote.

For although the first reaction in Ireland, Brussels and the rest of the European Union has been to say that the will of Ireland's voters must be respected, the wider political logic is that Irish voters are in effect saying no to the European Union, a will that can only be respected by other states freezing their ambitions. Ireland's voters have primed a bomb.

Thinking through the options shows just how big the bomb is. Eighteen states have already ratified the treaty, some for the second time. The first reaction of José Manuel Barroso, the president of the European Commission, was to ask the last eight member states, Britain included, to proceed with ratification. Gordon Brown has agreed; the final reading is on Wednesday and to stop the process because of the Irish vote would be unreasonable. The European Council of heads of state meets this week and will surely re-endorse that collective view. So the EU on 1 January 2009 will have a treaty that 26 states have ratified - but not the Irish. A new commission is to be appointed, supposedly on a new basis, as well as a new president, along with a strengthened foreign policy chief. So what is to happen?

What can't happen is that the treaty is scrapped, rewritten to accommodate changes to meet the will of Ireland's voters and then re-ratified in 27 countries. There are the practical questions of time and expense and there is no political readiness in the other 26 capitals to go through the whole interminable process again.

On top of these there is the political problem that the treaty can't be rewritten to accommodate specific Irish concerns because it already does; Ireland's 'no' campaigners told lies. The voters' great concerns had been met. There is a specific protocol that guarantees Ireland's neutrality and excuses it from membership of any joint European defence effort, if any surfaces. There is no possibility of Ireland being told to enforce abortion. And all states have autonomy over tax policy.

Crucially, the treaty contains a clause that states that do not agree to its provisions are required to leave the European Union. The existing treaty can certainly be made more obviously Ireland-friendly within its existing provisions, but beyond that, the EU will have to get tough and invoke the clause. It will have to ask Ireland to resubmit essentially the same treaty for a second referendum early in 2009, rather as Ireland held a second referendum over the Nice treaty in 2002.

If Ireland votes similarly again, then it will have to accept associate status in the EU and not be a member of its governing structures. The EU will proceed without Ireland.

Nobody wants this outcome and the language at this week's European Council will be carefully diplomatic. Senior British government sources have told me that the EU has to be 'respectful' of the Irish vote and not be portrayed in any way as a bully that disregards the popular will. This may be possible for a few months, but the clock is ticking. Decisions cannot be deferred. Sometime during the autumn, the President of the European Commission or the Irish Prime Minister must spell out Ireland's choices.

Irish and British Eurosceptics, in close alliance, will react in fury. I can see it now. This will be proof-positive of the Brussels elite's malevolence and anti-democratic intent. David Cameron's Tory party will say that Ireland is being treated disgracefully. I don't see how Cameron will be able to avoid a pledge to give British voters the same chance for a referendum on the treaty as the Irish, not least to strengthen the hand of the Irish 'no' campaigners in their second referendum. One of Europe's big states will be on Ireland's side when the Tories win power. They can take on Brussels bullies etc.

Battle is going to be joined in earnest because it must. Pro-Europeans everywhere must engage. We need this Europe - to fight climate change, to ensure security of energy and food, to underwrite our prosperity and to fight for our common interests.

The world needs it too. The EU is the citizens' friend. If it did not exist, Europe would have to invent something similar. Over the next few months, Europe's leaders are going to have to develop concrete initiatives to support these points and to present what they are doing as European and only possible because of the EU.

Maybe pro-Europeans can win Ireland's second referendum and then, in 2010 or 2011, our own. But referendums work best for the demagogue, the dissimulator and scaremonger, as Hitler and Mussolini, lovers of referendums, proved. Increasingly, Ireland and Britain are heading for the European exit and that could portend further break-up of the Union. Pro-Europeans look out.