Britain’s prosperity is also undermined by the very uneven spread of wealth. There are a handful of rich regions, with London pre-eminent, but much of the country is surprisingly poor. Nine of Northern Europe’s 10 poorest regions — including West Wales, Cornwall and Lancashire — are in Britain.

The prosperity we do have is highly dependent on our trade with Europe. Just about 44 percent of British exports, and more than half our imports, are with the European Union. Britain owes much of the foreign investment in its key industries, in cars and pharmaceuticals, precisely to the fact that the country provides an entry point for Europe. London’s financial and business services are a powerhouse of the economy, making up a third of Britain’s G.D.P., and cornering the union’s markets in this area.

Brexit means that we are about to willfully blow up all these ties. It will make all our trading relationships with our nearest neighbors more difficult and expensive. It will cut the flow of European Union migrants here. As Simon Tilford at the Center for European Reform has pointed out, Britain will be much less attractive to the foreign-owned businesses that generate half its exports once they cannot sell to the Continent without barriers. We will no longer be allowed to sell financial services freely in the European Union, and whatever access we negotiate for goods will have to be, as the union has made clear, substantially worse than what we have now.

The consequences will be serious and lasting. The organization that speaks for big business, the Confederation of British Industry, calculates that by 2020 the country’s G.D.P. could be 3 percent to 5.5 percent lower than if we had stayed in the European Union.

New trade deals with the rest of the world cannot make up for Brexit. According to the National Institute of Economic and Social Research, a leading think tank, leaving the single market will cost Britain a fifth of its trade in goods (and a quarter in services). Even if we negotiated free-trade agreements with 10 leading nations, including India, Brazil and the United States — a process likely to take years — that trade would make up for only about a quarter of what will be lost.

All this is before we take into account the expense, complication and bureaucratic replication involved in leaving the union; the years of negotiating with countries around the globe; the dozens of European regulatory agencies, overseeing everything from medicines to nuclear power, that Britain would now have to set up equivalents for; the need to recruit thousands of people to manage new and more extensive customs and immigration systems.