Those campaigning for the United Kingdom to exit the European Union claim that doing so would make their country both freer and richer. They assert that after the so-called Brexit, the UK could quickly negotiate a bespoke agreement with the EU that offers all the benefits of free trade without the costs of EU membership; strike better trade deals with other countries; and reap huge benefits from scrapping burdensome EU regulations. This is a delusion.

In fact, Brexit would entail huge economic costs for the UK. The uncertainty and disruption of drawn-out and doubtless acrimonious divorce proceedings would depress investment and growth. Permanent separation would reduce trade, foreign investment and migration, and hurt competition, productivity growth and living standards. And a divorce would deprive the UK of influence over future EU reforms - notably, the completion of the single market in services - from which it would benefit.

The London School of Economics' Centre for Economic Performance calculates that the long-term costs to the UK of lower trade with the EU could be as high as 9.5 percent of its GDP, while the fall in foreign investment could cost 3.4 percent of its GDP or more. Those costs alone dwarf the potential gains. The UK's net contribution to the EU budget amounted to only 0.35 percent of its GDP last year, and scrapping EU regulation would bring limited benefits, because the UK's labor and product markets are already among the freest in the world.