Much of the Brexit debate so far has focused on a perceived trade-off between controls on migration and membership of the European Union’s single market. If only the EU would abandon its dogmatic insistence on the free movement of people, the argument runs, then the obstacles to the UK staying in the single market would fall away. This assumption is by no means exclusive to Britain. There is a debate in Germany over whether the EU should back down on free movement as the price of preventing barriers emerging to lucrative trade with the UK, which in Germany’s case amounts to 4 per cent of GDP.

More surprisingly, Bruegel, the Brussels-based think tank, published a paper last month (whose authors included an adviser to the