David Cameron will be remembered by history as, among other things, the last leader of the Conservative party to support British membership of the European Union. The Prime Minister’s policy is not even to endorse membership under current arrangements but to negotiate a new deal with Brussels that voters would be asked to ratify.

A European referendum is now certain to be held at some point after the next general election. The crisis in the eurozone has provoked emergency reforms to accelerate political and financial integration among countries that share the single currency. Since Britain is not in that club, its status as a prime player in the wider union, equal in clout to Germany or France, is under threat.

All of the main British parties have accepted that a fundamental change in the relationship between the UK and Brussels would trigger a referendum. There has been much quibbling in the past about what constitutes such a change, but every conceivable outcome from the current round of institutional wrangling would qualify. The question for Cameron is whether he can fashion a new membership package and confidently endorse it without inciting regicidal fury in his party.

The theoretical outline of such a deal is simple. It would disentangle Whitehall and the private sector from a bunch of European regulations – mostly in the area of employment law and social protection – without surrendering trade privileges. Viewed from Paris or Berlin, this looks like a request to turn Britain into an offshore haven of low-cost production for export into European markets. It is about as appealing a proposition as “Del Boy” Trotter asking if he can park his Reliant Robin inside Selfridges to offload bootleg goods at cut-throat prices.

In Brussels, Cameron’s proposed negotiation to “repatriate” powers looks, at best, like a weird distraction from the more pressing matter of saving the single currency. At worst, it looks like an attempt to hijack the European reform agenda to placate a domestic audience, which EU leaders know from experience is implacable.

Satisfying Tory demands would mean completely rewriting the Lisbon treaty, the document that governs EU decision-making procedures. Its passage into law was so painful that many European leaders dread the prospect of reliving the experience, not least because they might then be constitutionally obliged to hold referendums of their own. The eurozone crisis makes some treaty change all but inevitable, but why, continental governments wonder, should the process be driven by British buyer’s remorse?

The UK already has privileges under the Lisbon Treaty, including the right of a blanket “opt out” from collective European justice and policing policy. On 15^th October, Theresa May, the Home Secretary, told parliament that the government plans to exercise that right. She also conceded that Britain would then have to start haggling its way back into those individual measures deemed vital in the battle against international crime (although she couldn’t say what they are). That decision raises the bizarre prospect of ministers from the Home Office and Justice Department lobbying their European counterparts to allow Britain back into arrangements that pool resources, when Downing Street’s policy is to find ways of doing the opposite. 

In Nick Clegg’s office, May’s announcement was seen as a panicky gesture aimed at soothing eurosceptic tempers in her party. The deadline for a final decision on the “opt out” is June 2014 and coalition negotiations European policing cooperation have barely begun. “They need to be a bit careful,” says one senior Clegg aide of Tory Brussels-bashing. “We aren’t signing up to a mass opt out until we’ve agreed what we are opting back into.” Lib Dems are especially keen on the pan-European arrest warrant, which, they say, enables UK authorities to nab serious villains overseas. Most Tories see it as a licence for continental judges to meddle in domestic legal affairs. The police and security services lean towards the Lib Dem view.

As ever, Cameron finds himself torn between the practical demands of government, whether in relation to his coalition partners or his European counterparts, and a party increasingly absorbed by dreams of what a Conservative majority administration might do. The ascendant strain of Tory thinking imagines Britain as a hub of buccaneering global enterprise, whose potential is stifled by nay-saying bureaucrats, mollycoddling welfare and outmoded deference to a decaying European project.

Ministers who came into government mildly Eurosceptic have had their attitudes hardened by the feeling that Whitehall channels the spirit of Brussels to thwart their ambitions. “When the civil service has finally run out of ways to block you, the last resort is to say you might run up against European law,” says one senior Tory advisor. At the apex of this supposed conspiracy is Sir Jeremy Heywood, the powerful cabinet secretary and chief whisperer of cautious counsel in the Prime Minister’s ear on EU matters. 

Flanked by Heywood and Clegg, Cameron must fish around in the tiny pool of concessions that Britain can realistically demand from Brussels without leaving the EU. He will catch nothing there to satisfy his party. Most Conservative MPs say their preferred option is to stay in the Union, only on more favourable terms. Yet there has never been a concession to Tory euroscepticism that was not followed immediately by demands for something more. The trajectory is clear and irreversible. Conservative hostility to Brussels is fuelled by the conviction that European integration and British competitiveness in global markets are mutually exclusive.

Cameron never wanted a referendum on Europe; he has been bounced into promising one. He is cagey on the detail because he has no strategy for devising a version of membership that his party might endorse. The tempo of Tory urgency on the subject is set not by the EU’s own reform timetable but by the prospect of humiliation at the hands of Ukip in 2014 elections to the European parliament.  

Senior cabinet ministers urge the Prime Minister to use the credible threat of exit as a negotiating tool to focus Brussels minds on granting British wishes, as if other European leaders haven’t yet noticed that the UK is already half-way through the door. Meanwhile, anyone in the Tory ranks with ambitions to one day hold the top job knows that a leadership contest would involve an arms race of anti-Brussels rhetoric. That makes Cameron the last Tory leader to get away with saying Britain should stay in Europe. It is entirely possible too that, under pressure from his party, he will be the first Prime Minister to declare that Britain could leave.