David Cameron’s position on whether there should be a referendum on Britain’s membership of the European Union is an entirely rational one. That isn’t to say he is doing the right things at European summits or has the right policy. It is simply an observation about the tricky position he is in, having to negotiate simultaneously in Brussels with fellow heads of government and in Westminster with his own party.

The two sets of demands are incompatible. In Brussels, the Prime Minister wants to influence the evolution of European institutions as they adapt to the single currency crisis. He needs to preserve British influence without signing up to any more political or economic integration. That balancing act gets harder when his continental counterparts think the UK is determined to sabotage their efforts and is, in any case, striding towards the exit. That is precisely the message that would be transmitted by a premature commitment to a referendum regardless of what comes out of current negotiations.

Tory Eurosceptics, meanwhile, argue that the prospect of a referendum will focus the minds of the PM and the rest of the EU, making it clear that the final deal has to be a good one for Britain ...or else. That view rests on the uncertain premise that other European countries desperately want to avoid a British exit. Diplomatic patience with the UK is running thin. Besides, seasoned observers of the Tory right (at home and abroad) recognise that the end game for many MPs is exit no matter what concessions are wrung from Brussels. Why should Angela Merkel or François Hollande offer David Cameron favours on the basis that it might help him control his party and buy a renewed mandate of the UK’s EU membership when they know perfectly well that it won’t?

But Cameron can’t simply tell his party to shut up and wait and see what he has negotiated before demanding a referendum. Tory MPs don’t trust his pledges on Europe and want some indication that the plebiscite they crave will materialise. So he has to indicate that he recognises the need for a vote without actually stating that there will definitely be one. Britain’s membership of the EU really ought to be ratified by a national vote but there isn’t much point asking the question until the terms of that membership are settled and they are now, thanks to uncertainty over the single currency, in flux. That is Cameron’s position and, as I say, it is reasonable given the political constraints he is under.

The most aggressively eurosceptic section of the Tory party, however, is minded to be unreasonable. I don’t mean that in the pejorative sense of ‘irrational’. I mean their patience has run out and they don’t want excuses. They feel Cameron has been given the benefit of the doubt on Europe in the past and has been flaky on the subject. (In fact he has been extraordinarily accommodating.) His promises to deliver something – maybe - at an unspecified point in the future are worthless currency in the Conservative ranks.

There is no great diplomatic advantage in sounding off about a referendum; if anything it weakens Britain’s negotiating position. Nor does the vague promise of a referndum do very much for non-aligned voters with other things on their minds. So the only point of even talking about a vote is as a gesture to the Ukip-leaning tendency and the only gesture that will satisfy them – a clear irreversible commitment to an in/out question - is one the Prime Minister cannot make. It is, in political terms, as if Cameron has pulled out a gun to look all macho eurosceptic when everyone knows he is firing blanks.