JPMorgan expects U.K. won’t delay the promised in out referendum on EU membership until 2017 but will put the issue to vote in late 2016 instead. Given the ruling Conservatives have only a small majority, any legislation could fail if euro skeptics within the party vote against it, suggesting the party leadership will want to get the issue resolved sooner rather than later. The prospect of a vote could weigh on the economy, again arguing for an earlier vote, so here are the pros and cons of Brexit simplified...

via HSBC

JPMorgan goes on to note that

Cameron initially said the vote would happen before the end of 2017 to give the govt time to renegotiate with other EU members

Since it seems unlikely Treaty revisions will take place within that timeframe and there’s limited room for the U.K. to secure any deep change, there may be strategic reasons to hold the referendum in late 2016, JPMorgan says

France, traditionally less sympathetic to U.K. concerns than Germany, has presidential elections in early 2017, suggesting there’s little chance of any significant concessions.