Bruce Anderson: There is no constitutional requirement to hold a referendum, but there is a moral one

There are two arguments against referendums. One of them is so superficial as to be intellectually worthless. The other is powerful but unfashionable. The first involves a lot of harrumphing about the sacrosanct nature of the British Constitution, which has no place for referendums. They are an alien implant hostile to its spirit.

That is easy to refute. Not being on paper, the British Constitution is hard to define and even harder to tear up. Its lack of paper-bound inflexibility also enables it to evolve, and it has done so successfully over the centuries. It is a simple matter to trace constitutional continuities from our day back to the era when men argued for royal absolutism based on divine right. Our constitutional history is a long disproof of the dictum that you cannot pour new wine into old bottles.

But this was by no means always a harmonious evolution. It involved crises – including a civil war – and fiercely contested modifications. By these standards, referendums are a relatively minor change. It could even be argued that judging by the number of occasions on which they have been used since 1975, they are becoming a constitutional convention. Even so, there is a strong case against them. It derives from the need to restrain democracy.

In America, the founding fathers were anxious to prevent democracy from degenerating into the mob rule of the ignorant and the propertyless. This explains the Supreme Court, the Senate and the Electoral College for the Presidency.

In the UK, similar fears about the malign consequences of democracy were expressed throughout the 19th century, until the time came when it was no longer electorally wise to do so. But the Tory party, full of demo-sceptics, was able to find three consolations. There was still a House of Lords and an independent judiciary. Moreover, the party which had opposed the Great Reform Bill turned out to be surprisingly good at winning democratic elections, and there was a further constraint. In Britain, we had a parliamentary democracy, not a plebiscitary one. In theory, decisions on great matters were left to those who had given them much thought, and who would act on the basis of deliberation, expertise and debate, unmoved by the fickle impulses of the multitude.

That theory, which was rarely expressed in public, might seem to bear little relationship to either the practices of the current House of Commons or to its composition. The almost complete elimination of deference from our national life has also undermined the status of the independent legislator. Imagine the reaction now if a politician were to claim that the gentleman in Whitehall or Westminster really did know best. But there are still grounds for insisting that vox populi is not vox dei and that, for all its faults, our legislature should take the decisions on monetary policy, the death penalty and the EU.

Yet there is a stronger, and indeed an unanswerable, case to be made on an even more important issue. Governments must not renege on solemn pledges which they have made to the voters. In 2004, Tony Blair gave such an undertaking. He declared that he would hold a referendum on the new European constitution. Indeed, he went further. He eliminated any remaining loopholes through which some future parliamentary weasel might try to make his escape.

This is what he said. "What you can't do is have a situation where you get a rejection of the treaty and then you just bring it back with a few amendments and say we will have another go." That is precisely what is now happening. On the basis of a few amendments, none of any consequence, Mr Brown is having another go. In so doing, he is breaking his word, for he cannot claim immunity from Mr Blair's commitment. A promise of such importance that is given by a Prime Minister must be binding on any senior member of the Government who does not resign.

When Mr Blair announced the referendum, he was performing a volte-face. This caused dismay, and not only among Labour ministers who, on the basis of the previous script, had been insisting that there would be no referendum. A lot of high-minded enthusiasts for Europe were upset, especially as Tony Blair's change of mind was first revealed by The Sun. For at least 100 Euro-fanatics in the greater Athaeneum Club area alone, this symbolised everything that they had come to deplore about Mr Blair's way of conducting business.

Some of them, who have still not forgiven Mr Blair, feel so strongly that they regard his referendum pledge as illegitimate. Peter Riddel of The Times, for whom EUness is next to godliness, appears to think that as Mr Blair should never have done what he did, the matter can be disregarded. If such a principle were adopted in the Law Courts, no criminal would ever be convicted. When it comes to the Prime Minister's word, to revive a 17th-century constitutional debate, friendly columnists do not possess the dispensing power.

The facts are inescapable. The Prime Minister gave his word. The Chancellor acquiesced. The commitment appeared in the Labour manifesto, thus ensuring that Europe hardly featured in the 2005 election. There is only one way in which Mr Brown could now justify his decision to break his word. He could call an election. Otherwise he must hold a referendum.

He can, of course, try to override that obligation by coercing a Bill through Parliament. He can assure his supporters that by the next election, the voters will have forgotten. But if he does that, let him never again prate away about his values. Let him accept that he will never again be entitled to lay claim to honesty, honour, integrity or truth. At the very least, will he please refrain from describing himself as a listening Prime Minister or rabbiting on about citizens' juries. If he had behaved this way in business, he would indeed be facing a citizens' jury, with 12 members.

Mr Brown's behaviour is incomprehensible. Even if he holds the duties and obligations of his highoffice in cynical contempt, one would have thought that he might have a sense of self-preservation. But if during months of arguments over Europe, the voters gradually become aware that the PM thinks that he can get away with breaking his word because their attention span is too short to remember, there will be a reckoning.

The Prime Minister clearly hopes that he can create dissent within the Tory party. He may find that this is another error. Events and personalities have moved on since the 1990s. It is true that Kenneth Clarke will be prepared to accept anything with a European label, however doubtful its provenance. But the issue of truth and the Blair commitment may well inhibit some of the other Tory Europhile grandees, who enjoy good personal relations with David Cameron. Despite their belief in Europe, they may be ready to acknowledge that this is not a European issue. It is a moral issue.