London Briefing/Chris Johns: The response to my last column has been such that the likelihood of a "No" vote in the UK's forthcoming EU constitutional referendum needs further explanation.

Outsiders seem to be both puzzled and fascinated by the determination of an overwhelming majority of British people to reject the new European Constitution.

The factors driving people to tell opinion pollsters they will vote no are many and varied. By contrast, the dwindling band of Europhiles tends to have a more homogenous view.

If the pollsters are right, and the constitution is soundly defeated, the consequences for the whole Union, not just Britain, will be profound.

Explaining the position of the europhiles is easy: their beliefs are based on a consciousness of Europe's bloody history and a mixture of hard economics and fuzzy idealism.

The primary goal of peace in Europe has - the Balkans aside - been achieved. Germany no longer invades France every generation or so. Europhiles often claim the successes of the EU to include much more than peace: the UK minister for Europe, Mr Denis MacShane, this week argued that the fall of communism, the Irish economic miracle and lots of other nice things happened because of the EU.

People like Mr MacShane often come over all misty-eyed when they talk about Brussels. Their remarks are always rooted in history and a belief that the spread of democracy to countries such as Poland, Spain and Portugal wouldn't have happened without the EU. One key weakness of their arguments (aside from being wrong) is that they are always backward-looking. Rarely do we hear about what the EU is going to do next. That, we suppose, is what the new constitution should tell us. I may have fallen asleep, but I couldn't find a recognisable mission statement in the draft document.

I suppose that Mr MacShane fears that if the EU stops going forward it must go backwards and that history will begin to repeat itself. Frankly, most British people simply don't buy this. Younger people think about Europe in purely pragmatic terms; all that vision stuff is simply meaningless for most people born after 1960.

In practical terms the EU can point to two big achievements: peace between Germany and France and the Common Agricultural Policy (CAP). Most British people think that the CAP is an abomination that enriches the wrong farmers, impoverishes Third-World agriculture and wastes the bulk of the EU budget.

The EU commission itself remains largely unaccountable, with more than a whiff of scandal still pervading the air. If the EU constitution is supposed to achieve anything it should be the remedy of these ills, but the sceptics are far from convinced.

Europhiles argue that the "Brussels bureaucracy" so reviled by the British media is in fact smaller than the BBC's or Birmingham City Council's and that 85 per cent of its budget goes back to member states. Britain's beaches have been cleaned up by the EU and, notwithstanding Gordon Brown's selfish claims to the contrary, our unprecedented period of economic growth is down to the sharing of sovereignty with our European partners.

All of this is terribly disingenuous of course and does the pro-European cause no good whatsoever. Making unrealistic claims for your cause means that you are treating your audience with contempt and they will return the compliment in spades.

Europhiles would be well advised to acquire some intellectual honesty and admit the political nature of their views - yes, they should make the case that a federal Europe is a good thing - and argue that nationalism is a poison that must be fought continuously.

Intelligent Eurosceptics find this line of attack the most uncomfortable. They prefer sticking to the easier stuff: what has Europe actually done for us? When the sceptics find themselves sharing the same views as Bill Cash and Gerry Adams, they do wonder about the company they are keeping. Pro-Europeans should exploit this unease for all it's worth.

Political conventional wisdom has quickly arrived at the conclusion that a rejection of the constitution will mean Britain has to leave the EU - or the EU finds a way to move ahead without us. A second referendum is becoming an increasingly remote possibility, mostly because the size of the likely "No" vote will make such an exercise futile.

The stakes couldn't be higher and Tony Blair is left pondering, yet again, the law of unintended consequences. Unless he manages to change a lot of minds, EU countries with close economic ties to the UK should start thinking about the post-referendum world.

The Irish authorities would be well advised to start considering their response to a decision by the UK to leave the EU.