BRITISH PRIME Minister Theresa May’s decision to hold early elections in June, reversing what had been a firm public position, surely reflects her recognition that Britain’s exit from the European Union will be far more complicated and painful than voters were promised when they supported it in a referendum last year.

The “leave” campaign promised that migration from the other 27 E.U. countries would be curtailed and the jurisdiction of E.U. bureaucrats and the European Court of Justice abolished, without damage to an economy that is heavily dependent on free trade with Europe. In fact, as Ms. May has begun to acknowledge, regaining control of Britain’s borders will mean a costly exit from the common market. In addition, Brussels could hand Britain a bill for tens of billions of dollars in residual payments, and a new trade deal could take years to negotiate.

By 2020, when the election would have been held under the usual schedule, Britons are likely to be suffering the heavy costs of a decision that so far has not had much practical impact. A vote now could extend the term of Ms. May and the Conservatives to 2022, giving them more time to manage the fallout. More importantly, it is likely to substantially increase the government’s small, 17-seat majority in the 650-member House of Commons, thanks to the abysmal state of the opposition Labour Party.

Unfortunately, the election will strand many of the 48 percent of voters who opposed and, according to opinion polls, still oppose Brexit. Under far-left leader Jeremy Corbyn, Labour is weakly ambivalent on the issue. The small Liberal Democratic Party has taken a firm stand against leaving the union, but while it is expected to gain seats, for now it is polling at around 10 percent and appears unlikely to stop a Conservative landslide.

For Ms. May, who replaced David Cameron nine months ago and has not yet won her own election, the most important question may be not the size of her margin over Labour, but the composition of the new Conservative parliamentary group. To succeed in negotiations with E.U. leaders she will need the flexibility to overrule party hard-liners who will oppose any concession on borders and regulation; that would be difficult in the current Parliament. A more moderate majority will be essential to deals preserving British access to the European market in key areas, such as finance and auto manufacturing, without which the economy could be severely damaged.

For now, Ms. May remains carefully vague about the terms of an exit agreement. Apart from saying in a January speech that control over migration and escape from the European Court of Justice were priorities, and a departure from the single market and customs union a consequence, the prime minister has been unclear on a range of issues, such as whether Britain will consent to pay the huge exit bill that some E.U. officials say it will owe. No doubt she will be pressed during the campaign to tell her supporters more clearly what they are voting for; but for the same reasons she decided to call an election, Ms. May will likely demur.