Britain does not have the physical infrastructure or bureaucracy to manage such checks, nor do some of its European trading partners. The British government has said it will wave trucks through the border, at least for the first several months, but the European Union will not. So trade would slow down, particularly on the Continental side, hurting merchants of perishable goods and manufacturers; some products could be barred from crossing borders; and experts predict higher prices and shortages.

In addition, Britain has no trade treaties with most of the world, including the United States, because it has operated under the European Union’s treaties, and negotiating new ones can take years. In a no-deal Brexit, the existing arrangements would abruptly cease to exist and Britain would default to rules set by the World Trade Organization, upending the way every importer, exporter and regulator in the country operates.

Economists and the government say that leaving without a plan in place would do severe economic harm. But the most ardently pro-Brexit lawmakers in Britain insist that the cliff edge is nothing to fear, and preferable to a long delay.

What is all the talk about elections?

There is at least one set of elections approaching that could affect Brexit, and others are possible.

European Union member countries are scheduled to hold elections for the European Parliament from May 23 to 26.

But if there were a two-year delay for Brexit, as some European officials have proposed, then Britain would be in an awkward position: It would be required to participate in elections, which the major British political parties do not want to take part in, to govern a bloc it intends to abandon.

The exit dates set in Brussels were chosen with these European elections in mind: May 22 is the day before Britain would have voted, and April 12 is the last day it could start preparations to hold a vote.