David Cameron has embarked on a bold, possibly reckless, strategy to sort out Britain’s troubled relationship with the EU. He promises that, if re-elected, he will renegotiate membership terms, then hold an in-or-out referendum in 2017. The prime minister’s problem is that large parts of his Conservative party expect the renegotiation to deliver much more than his EU partners are likely to offer.

Many Tories believe EU leaders, faced with a UK exit, will give Mr Cameron enough to enable him to argue credibly that he has changed the terms of UK-EU relations. They count on Angela Merkel, Germany’s chancellor and the prime minister’s ally, to fix the rest of the EU.

However, this optimism is built on a shaky foundation: the belief that the euro crisis will force the EU to revise its treaties in time for the 2017 referendum. A new EU treaty needs the signature of every member state, so would give Britain the leverage to extract concessions from its partners.