GREECE has long seemed a decent bet to crash out of the euro zone, and perhaps also the European Union. But during the current election campaign in Britain, Britain's exit (or a "Brexit") from the EU has also started to seem a real possibility. David Cameron, the prime minister, has promised an in/out referendum vote on Britain’s EU membership by the end of 2017 if the Conservatives remain in power on May 7th. Several other smaller parties, notably the UK Independence Party (UKIP), are campaigning specifically to leave the EU. Moreover, polls show that any referendum would be an extremely tight one. Why might Britain be leaving the EU, and how would it happen?

Even at the best of times Britain has always been a semi-detached member of the EU. The first post-war Labour governments turned down the opportunity to participate in the negotiations that led to the birth of the forerunner of the EU in the early 1950s. Since then Britain has often been more sceptical of the European project than committed to it; the country has been called “the awkward partner”. Britain eventually joined what was then called the European Communities under a Tory government in 1973, as Europe seemed to be doing so well economically. But the next Labour government quickly held a referendum on membership in 1975. The majority, on that occasion, voted to stay in, but over the past few decades British governments have kept their distance as others within Europe pursued "ever closer union". Britain did not join the single currency, and it is not a member of the Schengen passport-free travel zone. Traditional hostility to the EU has increased in recent years with the arrival of hundreds of thousands of east Europeans (quite legally, under EU rules) to find work, which has caused parties such as UKIP to assert that they are hogging an unfair proportion of the housing, school-places and health services that should go to Britons.