LONDON — Following a tumultuous, unpredictable snap election, Prime Minister Theresa May, a Conservative, appeared on the verge of losing her overall parliamentary majority, according to a national exit poll released just after voting ended on Thursday night.
If confirmed by the actual vote count, the result would be a major setback for Mrs. May. She called this election three years early, expecting to cruise to a smashing victory that would win her a mandate to see Britain through the long and difficult negotiations with the European Union about withdrawing from the bloc.
According to the exit poll, Mrs. May may have lost the extraordinary gamble she made in calling the election — and Britain may be headed for a hung Parliament, in which no party has a majority.
Poll results found that the Conservatives would remain the largest party, but they were projected to win only 314 seats, down from 331 won in 2015, and 12 votes short of a majority.
The opposition Labour Party, led by Jeremy Corbyn, appeared on track for 266 seats, up 34. The Scottish National Party was projected to be down to 34 seats from 56, while the centrist Liberal Democrats were projected to win 14 seats, up from 8 in 2015.
Stunned party officials expressed some skepticism about the exact numbers of the exit poll, noting the projections showed that more than 70 seats are very close and hard to call. Analysts also said that while the exit polls are generally reliable, predicting a precise margin is difficult. In 2015, the exit poll had the Conservatives at only 316 seats, short of a majority; the actual result gave them a majority, with 331.
Even so, it is clear that Mrs. May will be badly hurt. And it may mean that neither major party is able to form a working majority. If so, another election may be in the cards.
The former chancellor of the Exchequer, George Osborne, said that if Mrs. May loses her majority it would be “completely catastrophic” for her and the Conservative Party.
“It’s difficult to see if these numbers were right how they would put together the coalition to remain in office,” he said. “But equally it’s quite difficult looking at those numbers to see how Labour could put together a coalition, so it’s on a real knife edge.”
Clearly, though, Britons confounded expectations and the betting markets once again. Negotiations to begin withdrawal from the European Union are scheduled to begin in less than two weeks, even as the shape of the future British government now appears uncertain.
The final outcome is not likely to be known until early Friday morning. But the British pound fell sharply after the exit poll projected that the Conservatives would not win a majority.
Within seconds of the exit poll being broadcast, the pound lost more than 2 cents against the dollar, falling from $1.2955 to $1.2752.
Given the two terrorist attacks that took place during the campaign, security was tight on Thursday as Britons voted, with a heavy police presence.
Maria Balas, 28, a waitress, said security was the prime issue. “England is under attack and at this time we need a strong leader more than ever,” Ms. Balas said after casting her vote for the governing Conservative Party. “I don’t like Theresa May, and I wouldn’t have bothered to vote if this election was all about giving her more power to take us into the mess of Brexit, but now we are dealing with a security crisis and I think she is the most qualified person in the running who can deal with that.”
In London’s eastern borough of Hackney, however, young people seemed more concerned about future job prospects.
“The Tories only care about the rich and their interests,” said Luke Wright, 26, who earns £7.50 an hour, or about $9.70, working at a stationery shop.
“I have computer skills that could get me a much higher paying job, but they are given to people who could afford university fees and living costs,” he said. “If Labour won I’d have a chance to make more cash and get out of this job that I’m overqualified for.”
Tensions were high in the Stamford Hill area of East London on Thursday after a man started shouting at a group of Jewish pedestrians, “Allah, Allah, I’m going to kill you all.” He was quickly detained by the police.
Mrs. May, 60, rolled the dice on April 18 when she broke her promise not to call an early election, three years ahead of schedule, but did so only because she believed the dice were loaded in her favor.
She went into this election with a 20-point lead in most polls and a working majority of just 17 seats in the 650-seat House of Commons, the lower house of Parliament. In the 2015 election, the Conservatives surprised the pollsters by winning 331 seats, a majority of 12.
While she was personally against Britain’s exit from the European Union, or Brexit, in the June 2016 referendum, the vote in favor caused David Cameron to resign, and she emerged as a kind of accidental prime minister.
But she promised voters that she would honor the results of the referendum, using her reputation for toughness “to get the best deal for Britain.” If the Europeans proved unforthcoming, she promised that “no deal is better than a bad deal,” appealing to the British sense of nationalism and self-reliance.
That included a pledge to end unfettered immigration, an effort to reach out to the nearly 13 percent of voters in 2015 who voted for the U.K. Independence Party, whose platform was anti-immigrant and pro-Brexit. Many of those voters, especially in the West Midlands and the north, were traditionally Labour supporters, but with the collapse of UKIP, many of them were thought to lean to the Conservatives.
That meant Labour-held seats were ripe for the picking, especially since northerners were not enamored of Mr. Corbyn, 68, a far-left urbanite. He seemed weak on defense and security, shaky on economic management, passionate about places like Venezuela and Nicaragua, had once had strong sympathies for the Irish Republican Army and liked to make jam.
And the centrist Liberal Democrats, who emphasized rerunning the Brexit debate in a second referendum, were getting very little traction. While the business elite were laser focused on the issue of Britain’s future relationship with the European Union, opinion polls showed that the general population had moved beyond that and cared more about domestic issues.
Strangely, for such an important issue, the economic impact of Brexit barely figured in this campaign, perhaps because its strongest effects, should they materialize, will not be felt for some time.
Mrs. May and the Conservatives ran an unusually personal campaign, trying to emphasize the differences between her and Mr. Corbyn on questions of leadership, reliability, economic competence and security, helped by the rabidly anti-Corbyn, pro-Brexit tabloid press.
But the Conservatives did not count on her poor performance on television and shaky presence on the campaign trail, particularly when confronted by hostile questioning. Rather than “strong and stable,” as her mantra went, Mrs. May could seem brittle and querulous, repeating slogans rather than dipping into substance.
Her party’s manifesto was also vague on figures, and her effort to find more funds for social benefits backfired when she announced, with little consultation with her cabinet colleagues, her intention to charge the better-off more for extended social benefits, saying that old people could keep assets up to 100,000 pounds, including the value of their homes. Quickly labeled “the dementia tax,” it damaged her badly with the Conservatives’ main supporters: older Britons.
Worse, it forced her into another U-turn on policy, the first during a campaign in recent memory. That and her poor campaign performance damaged her image, at least to some degree, within her own party and the country itself.
“Theresa May doesn’t look happy on the campaign trail,” said Mark Wickham-Jones, professor of political science at the University of Bristol. “And Labour have proved quite effective at chipping away at things like her reluctance to debate.”
At the same time, Mr. Corbyn, who survived an attempt last year by his own members of Parliament to unseat him as Labour leader, had a very good campaign. Appealing to the young, especially in the big cities, Mr. Corbyn ran on a platform promising more social justice, free college tuition, more money for the National Health Service and welfare, the re-nationalization of the railways and utilities, and much higher taxes on corporations and those earning over £80,000, about $104,000, a year.
His performances on television were calm and avuncular, with a touch of humor. And as the campaign wore on, he appeared to win back the support of most Labour voters in 2015, plus some Liberal Democrats and Greens.
The polls narrowed. But the Conservatives never lost their lead in any major poll. And party professionals on the ground, especially in marginal seats in the Midlands and the north that the Conservatives had targeted, reported continuing resistance to Mr. Corbyn as a credible prime minister.
The campaign was also marred by two terrorist attacks that caused numerous casualties, in Manchester on May 22 and then, last Saturday, in London. These also seemed to work against Mrs. May, at least at first. As home secretary for six years before becoming prime minister, she was criticized for the failures of the security services to stop the plots and for supporting cuts in beat policing.
Yet, late polling indicated that she benefited from her tough response, especially after the London attack, promising new counterterrorism legislation, and had widened the gap with Labour at the end.
She is trying to break away from firm promises that limit her freedom in office, Mr. Wickham-Jones said. “But I think there are question marks about her leadership of the party, and if she comes out with a majority of 40 or even 50, I think some Conservatives are going to say, ‘Was it really worth it?”’
The candidates spent the last day of official campaigning racing around the country — Mrs. May by jet, Mr. Corbyn by train. “They underestimated us, didn’t they?” he told a rally in Glasgow.
Mrs. May, who refused to debate Mr. Corbyn face-to-face throughout the campaign, appeared in carefully managed settings. But in an indication of her party’s confidence, she continued to campaign in seats the Tories think they can win, while Mr. Corbyn tended to stick to seats Labour could not afford to lose.
In a last appeal to voters, Mrs. May said: “If we get Brexit right, we can build a Britain that is more prosperous and more secure, a Britain in which prosperity and opportunity is shared by all,” asking Britons to trust her to “knuckle down and get the job done.”
Correction: June 8, 2017
An earlier version of this article misstated the number of parliamentary seats held by the Scottish National Party heading into the election. It was 56, not 59.