Blair will call a referendum for many reasons. One is that he has no option

Another failure of deductive reasoning. Logic, you may have noticed, is not a discipline in which we journalists consistently excel. This self-critical thought has been brought on by the news that Tony Blair is already three-quarters of the way round the curvy bit of the U in his policy on a referendum on the European Union constitution.

Another failure of deductive reasoning. Logic, you may have noticed, is not a discipline in which we journalists consistently excel. This self-critical thought has been brought on by the news that Tony Blair is already three-quarters of the way round the curvy bit of the U in his policy on a referendum on the European Union constitution.

It should have been possible to write a column a month ago suggesting that a referendum was a near-certainty. When Jose Maria Aznar's conservative party lost the election in Spain, immediate attention focused on whether this was appeasement of terrorism by the Spanish electorate. Rather less attention was paid to the consequences of the election of the socialists, whose leader, Jose Luis Rodriguez Zapatero, finally took office on Friday. One of those consequences was the reversal of the Spanish government's opposition to the draft EU constitution that Aznar had blocked in December. When the Poles too indicated their willingness to compromise, the constitution, which British pro-Europeans had pronounced dead, suddenly sat up in the bath. At the Brussels summit on 26 March, it was agreed to hasten negotiations to produce a deal on the constitution by June. This was, of course, the last thing Blair wanted - and most pro-European realists in Britain shared his dismay at having to fend off, once again, demands for a referendum on the text from Rupert Murdoch's four newspapers plus the Daily Mail, the Daily Express, The Daily Telegraph - oh, and the Conservative Party.

At that point it should have been possible to report that Blair would announce he had been in favour of a referendum all along. It would have required only a few moments' thought about a small number of facts. These facts are well known, not least because the arguments about a referendum raged for more than a year while the constitution was being drawn up by the Convention, chaired by Valéry Giscard d'Estaing. Above all, it had become obvious that, if the constitution were agreed and the Bill to ratify it presented to Parliament, there would be a clear majority in the House of Lords for inserting an amendment requiring it to be approved in a referendum. That Labour peers are in a minority in the Upper House is one of the fixed but often neglected features of political life. On this issue, they would be outvoted by anti-European Conservatives, pro-"democracy" Liberal Democrats and a mixture of crossbenchers.

The House of Lords is, therefore, a road block. Not an absolute one, because it only has the power to delay Bills for about a year. Blair could put the EU constitution in Labour's manifesto for the 2005 election, and then appeal to the convention that the Lords do not obstruct manifesto policies. But it is an awkward issue to fight over, and it would help the Conservatives make Europe an election issue on terms unfavourable to the Government. While the pro-Europeans know the arguments against a referendum on this particular question, the demand to "give the people a say" tends to carry all before it, especially in the simplified politics of the hustings. It is no use protesting that the constitution is just a tidying-up exercise, or that the Single European Act and Maastricht Treaty transferred more power to Brussels, or that many who are calling for a referendum see it as a step towards leaving the Union. The problem is that, if the tidying-up exercise is called a constitution, it ought to be capable of being put to a referendum. Had that been the position at the start, the Giscard d'Estaing Convention might have come up with something better than the burbling 335-page document that it managed. There is nothing like the prospect of a democratic vote to focus minds.

That point is related to the secondary factor that should have made it obvious that Blair was about to change his policy. If he promises a referendum on the EU constitution, his bargaining position in the final round of negotiations on the text is greatly strengthened. He can say to the French and the Germans: "I'm sorry, if I concede that point it will make it impossible for me to win my referendum, and then the anti-Europeans really will be replete with Schadenfreude."

The final consideration has always been the risk of losing a referendum, which remains significant, but the delay in finalising the constitution means that the referendum could be held after the general election, instead of just before it. Opinion polls show large majorities against the constitution, albeit on the basis of knowing next to nothing about it except that most newspapers say it is the end of Britain as a sovereign nation. But Blair, who has thought for the past 10 years about how to win a referendum on a different European issue, namely the currency, can see how this one might be more winnable.

It is, after all, hard for the anti-Europeans to substantiate their argument that the constitution transfers powers from member states to the Union because, er, it doesn't. The basic shape of this referendum would, therefore, be more like that of 1975, an affirmation of the status quo, than one on the euro, which would be a definite step towards a closer union.

What the decision comes down to, then, is the balance between making Europe an election issue and the risk of losing a referendum, which would not be held until after the election. Unlike a vote on the euro, losing a referendum on the constitution would not end Blair's European dream. Everyone knows that the EU can operate without the constitution - as it will do for at least 18 months after the 10 new members join in two weeks' time. The constitution is supposed to make it work better with 25 members, but the larger Union can function without it.

Nor is Blair's prestige tied to the constitution as it would be to the euro. For all the talk of U-turns, he has already found his reverse gear once on this issue. When Andrew Marr was editor of The Independent, he filled the front page with a draft Constitution for Europe, on the plausible grounds that the European vision needed to renew its democratic legitimacy. Blair, then on the threshold of power, was magisterially dismissive, and once he became Prime Minister continued to describe the idea as unnecessary until the French and Germans decided otherwise.

It would be embarrassing for the Prime Minister to lose a referendum, but the final calculation is that it may not come to that. Eight other countries are committed to holding referendums on the constitution if it is agreed, including Ireland and Denmark, both of which have rejected European treaties in referendums before.

Therefore we did not need Blair to drop hints to Rupert Murdoch, who then tipped off the political editors of The Sun and The Times, which is apparently how the story came into the public domain. We could have worked it out for ourselves. But we didn't, even though, as Hercule Poirot says, the clues were staring us in the face.