The mistake David Cameron made was in promising a speech on Europe at all. With the eurozone convulsed by crisis and Euroscepticism leaching away political support at home, such a pledge could only raise expectations. Months of hints, delays and postponements only upped the ante. By this morning, when the speech was finally delivered, the Prime Minister was backed into a corner from which only something truly radical could extricate him.

And radical it was. Should the Conservatives win a majority in 2015 then the resulting government will hold an in/out referendum on EU membership by the end of 2017. Thus Mr Cameron fired the starting gun for the next election. He also, despite all the protestations to the contrary, put Britain's place in Europe in doubt for the first time. The dangers attendant upon such a move can hardly be overstated and the attempts to dress it up as firm leadership – confronting an unavoidable issue so as better to shape the debate – are far from convincing. Indeed, Mr Cameron has allowed his own political pressures to cloud his judgement of Britain's best interests.

In fact, the Prime Minister's message was less of a triumph of Euroscepticism than its reception by – among others – Ukip's Nigel Farage might suggest. Mr Cameron's analysis of the EU's shortcomings was couched firmly in terms of his hopes to remedy them. He stressed his desire for Britain to remain at the heart of the Union. He also spoke out against the hackneyed over-simplifications of the naysayers: a solo Britain would not flourish as a non-EU Norway or Switzerland does, he rightly pointed out, and our geopolitical influence would be materially eroded.

Taken in isolation, Mr Cameron's plan for a new-look Europe also has some appeal. In the lean and hungry trading bloc of the Prime Minister's imagination, the only non-negotiable is the single market. Everything else is up for grabs. "Cumbersome rigidity" is out, replaced by "speed and flexibility", a relentless focus on global competition and a political constitution that welcomes members' diversity rather than trying to "snuff it out".

So far, so good. Except that Mr Cameron is attempting to reconcile the irreconcilable – to steal the thunder of his political rivals, to knit together a Tory party threatening yet again to tear itself apart over Europe, and to do so with a vision of a re-shaped EU with which even sceptics might grudgingly agree. In practice, his proposals collapse under the weight of their own contradictions.

Pressing ahead with the single market is simply not consistent with a free-for-all of renegotiations and opt-outs. Moreover, if the EU suffers from sclerotic complexity now, the unwieldiness of a pick-and-mix agglomeration of trading areas, travel zones, military agreements, and monetary union can barely be imagined. The politics is trickier still. The "European partners" with whom Mr Cameron must negotiate do not share his conception of Europe as primarily a trading pact. Nor are they inclined to allow Britain to treat Brussels as a buffet from which it may choose what it fancies and leave the rest. The threat of a British exit will not help. It is true that much of Europe, including both Germany and France, are keen for us to remain in the Union. But not at any price.

The only shred of consolation is that, behind the Prime Minister's deftly crafted clarity, much wriggle room in fact remains. The biggest "if" is the outcome of the 2015 election. It is far from certain that the Conservatives will win the outright majority today's plans require. Mr Cameron has also avoided nailing his own colours unequivocally to the mast. He talked airily of his commitment to Britain in Europe. He talked of powers on the environment, social affairs and crime handed back to member states. But he carefully eschewed specifics – either as to what he hopes to achieve, or as to how he will vote in the promised referendum if he fails.

Hopes of a fudge to come are, at best, cold comfort. Five years of uncertainty on so fundamental an issue would be damaging at any time. With the economy flatlining, they are potentially catastrophic. That the Prime Minister's latest move is so at odds with his avowed intention for Britain to be a hub for global investment only underlines his desperation.

What now? The public mind is, thankfully, far from made up on Europe; opinion polls record a sharp rise in pro-EU sentiment in recent months. It is up to Britain's Europhiles to rise to the challenge. That means the business community making clear what is at stake. It means Nick Clegg shaking off his Coalition loyalties. It means Ed Miliband sticking to today's claim to be against an in/out referendum, even as pressure builds in the run-up to 2015.

The irony is that Mr Cameron himself counselled against hasty action today. If only he had followed his own advice. Instead, as Mr Farage gleefully noted, the genie is now out of the bottle. Britain is the loser.