If the UK leaves the EU the impact would depend on the new relationship between the UK and the EU. We consider five models. Those at the extremes in terms of proximity to the EU are unlikely. The Norwegian model, involving membership of the European Economic Area, would not give the UK the political flexibility required to justify Brexit. By contrast, a much looser model in which the UK trades with the EU on a most-favoured nation basis would give flexibility, but seriously jeopardise trade and investment. The most likely models are either a Swiss-style series of bilateral accords governing access to specific sectors of the single market or a comprehensive FTA. Either would require prolonged negotiation followed by compromises and still impose sizeable costs. A lack of clarity over what would replace EU membership is just one reason why the path to Brexit - and beyond - would be long and uncertain, taking ten years or more.