I am clutching in my hot, trembling hands the most extraordinary document I have come across in eight years of Euro-politics. It is a letter from the German Chancellor, Angela Merkel, to her fellow EU heads of government. In it, she proposes a scheme to bring back the European Constitution under a new name - or, as she artlessly puts it, "to use different terminology without changing the legal substance". Now this, in itself, is not surprising. Many of us have suspected all along that the Eurocrats would try to bring back their constitution surreptitiously: I have written as much in these pages. What is shocking is the brazenness. Mrs Merkel flagrantly admits that she wants to preserve intact the content of the constitution, making only "the necessary presentational changes".

These changes mainly involve dropping paragraphs that the voters don't like, and which are in any case unnecessary because they restate what is in existing treaties. Thus, Mrs Merkel suggests excising the reference to the primacy of EU law. Since this concept has been part of EU jurisprudence since 1964, she reasons, there is no point in rubbing people's noses in the fact by spelling it out. She also proposes scrapping the reference to the EU's symbols. Again, not a single 12-star flag will be hauled down as a consequence. The bands will still strike up Beethoven's Ninth, bringing a lump to Euro-enthusiast throats (I'm afraid that that stirring tune now has much the same effect on me as it has on Alex in A Clockwork Orange, and for the same reason - bad connotations). The change will be, as Mrs Merkel puts it with such admirable frankness, presentational. Similarly, she has a clever wheeze to "replace the full text of the Charter of Fundamental Rights with a short cross-reference having the same legal value". And so on.

The leaking of this letter is calamitous for the Euro-federalists. Their whole strategy depended on obfuscation, complexity and voter fatigue. The electorates of Europe might sense that their leaders are up to no good but, so far, they have not been able to hang their doubts on anything specific. Now, though, they have it in black and white: they are to get the same constitution as before, but without the promised referendums. Think, for a moment, about how scandalous this is. After all, Labour's commitment to a plebiscite did not come as an afterthought. It was central to that party's election strategy. There was a time, back in 2004, when it looked as though Europe might again dominate British politics, greatly to the detriment of the governing party. People could see that Brussels was engaged in a huge power-grab. They could see, too, that other countries were offering referendums. The Tories and the Lib Dems were demanding the same right for Britain.

Tony Blair feared, with good reason, that, if he did not allow a referendum, voters would treat the 2004 European election and, worse, the 2005 general election as surrogate referendums. Returning from the Caribbean, tanned, fit and lean, he suddenly announced that he would, after all, let the people decide. We Tories were left opening and closing our mouths like Appalachian yokels. Mr Blair's announcement deprived us at the last minute of what was to have been our main argument. I remember, as a Euro-candidate in 2004, having to pulp whole forests of redundant campaign literature. We duly went down to the worst defeat the Conservative Party has ever suffered - worse even than the catastrophe of 1832.

Having promised a referendum in two manifestos, and having won office on that basis, Labour will find it hard to explain why it now wants to rat. The publication of the Merkel letter makes it impossible to pretend that the new text is substantively different from the old one. No doubt ministers will try, essaying all sorts of sophist arguments to the effect that treaties are different from constitutions, and that the EU is already doing most of the things that the sceptics complain about. None of it will wash, though.

I hope I never have to give an interview like the one poor Geoff Hoon gave to The World at One last week. His own mother, had she been listening, would have thought him a terrible fibber. "What was different about the constitutional treaty," stammered the hapless Europe minister, "was that it altered the basic relationship between the European Union and the member states, and therefore it was appropriate to have a referendum." How painful to re-read those words in the context of the Merkel letter. Let us be clear: the European constitution amounts to a revolution in how our country is governed, perhaps the most far-reaching since the civil and religious upheavals of the 17th century. This revolution is taking place not as the result of popular insurrection or foreign occupation, but because the governing party is abusing its majority.

Labour may get its way, in the narrow sense of ramming the new treaty through without a referendum. But it will pay a heavy price in damage to its reputation, as will the Euro-integrationist cause more widely. "Vencerán, pero no convencerán," as Miguel de Unamuno told the Nationalist leaders at the beginning of Spain's Civil War: you'll conquer, but you won't convince. Parliament is not the owner of our freedoms, but their temporary and contingent custodian. If Labour MPs want to give those freedoms away in perpetuity, they should have the decency to ask us first. If they win, I promise to accept the result with as much good grace as I can muster. But if they go back on their manifesto promise, they won't deserve to be forgiven.