In what would be a shocking repudiation of Prime Minister Theresa May, British exit polls Thursday suggested the ruling Conservatives were on course to lose their majority in Parliament in an election Mrs. May called to cement her power and boost her bargaining leverage ahead of tough negotiations over exiting the European Union.

While the final results were uncertain late Thursday, it seemed clear that Mrs. May’s bid to build up her slender majority had backfired badly and that the Labor Party and its beleaguered leader, Jeremy Corbyn, had far surpassed the expectations of just two months ago.

While the Conservatives were on course to remain the single largest party in the House of Commons, the exit polls pointed to a “hung Parliament” that would require Mrs. May to cobble together a coalition to stay in power or even clear the way for Mr. Corbyn to try to put together a governing coalition of his own.

In what pundits just seven weeks ago were saying was an election that could decimate the leftist Labor Party, the BBC projected early Friday that Labor was on track to gain 29 seats in the all-powerful House of Commons, mostly at the expense of the Scottish National Party (SNP). The Conservatives would lose nine.

Responding to the numbers after retaining his seat, Mr. Corbyn said that Mrs. May should resign “and make way for a government that is truly representative of this country.”

Even with the results still trickling in, the British pound was falling sharply on world currency markets amid fears that the vote might not produce a clear winner or leave Britain with an unstable coalition ill-equipped for the Brexit talks. The knives were already coming out for the prime minister.

‘Lasting shock’

The BBC projection was a little less unfavorable to the Conservatives than were the first exit poll projections, released shortly after the voting ended, which had Labor gaining 34 seats. But the polling was sufficient to have analysts writing Mrs. May’s political epitaph.

Craig Oliver, communications director to Mrs. May’s Conservative predecessor David Cameron, predicted a “deep and lasting shock” to the party and to Mrs. May.

“It was the biggest gamble a politician has taken for a long time, and if that exit poll is right, it’s failed,” he told Sky News.

However, uncertainty swirled into the night over the results of a vote that was supposed to be a cakewalk for Mrs. May but had turned into a political minefield down the stretch after Islamist-inspired terrorist attacks that hit Manchester last month and London last week.

Analysts said Mrs. May couldn’t have predicted the twist, which shifted public focus away from her perceived image as a far more decisive leader than Mr. Corbyn and instead thrust media attention onto the prime minister’s record as Britain’s home secretary — the Cabinet position roughly analogous to the U.S. attorney general.

The attacks raised difficult questions about the ability of the woman who had been Britain’s top law enforcement official to manage the jihadi threat.

Beyond whether the Conservatives can cobble together a government, there remains the possibility that Mrs. May might not be able even to keep her post as party leader after personally deciding on April 11 to call the surprise snap election.

At the time, Mrs. May’s Conservatives held a 20-point lead over Labor in the polls.

But the BBC projection showed the Conservatives winning 322 seats in the House of Commons. The House of Commons has 650 seats, meaning a party would need 326 to form a government.

Labor’s unofficial total was 261 seats, with the SNP taking just 32, a loss of 24 Scottish seats it had held prior to the election.

The Liberal Democrats were projected to win 13 by the BBC, up from the current eight-seat contingent. But their party leader won’t be sitting in the House of Commons, as Nick Clegg lost his Sheffield Hallam seat in northern England to Labor. Mr. Clegg had been deputy prime minister when his party was part of a coalition government from 2010 to 2015.

The BBC projected other parties to win 22 seats, mostly the Welsh nationalists and the local parties in Northern Ireland, where the major parties generally don’t run.

The final count for the last district is expected sometime Friday morning.

Uninspired campaign

Despite her advantages, Mrs. May, 60, proved to be an uninspiring candidate whose campaign was marked by a string of unforced errors.

Besides the terrorist attacks, Mrs. May wasn’t able to take advantage of the Brexit talks, which she hoped to make a centerpiece of her campaign.

Unburdened by expectations, Mr. Corbyn proved far more effective on the stump, exploiting Conservative errors on domestic policy and even using the terrorist attacks to ding Mrs. May for nationwide cuts to police forces as part of the Conservative austerity program.

Still, the polls going into the vote suggested that the Conservatives had done enough to at least break even. Many saw the election coming down to turnout among young voters, who were seen as much more likely to lean left and vote for Mr. Corbyn’s Labor Party.

Analysts compared Mr. Corbyn’s plight to that of Sen. Bernard Sanders of Vermont in last year’s Democratic presidential primary in the U.S., but the exit polls suggested Mr. Corbyn had done a far better job turning out the young voters who lean to the left but do not turn out at the polls in the same numbers as conservative voters.

Desmond Lachman, a resident fellow with the American Enterprise Institute, pointed to “serious missteps” by Mrs. May.

“In a manner reminiscent of Bernie,” Mr. Lachman wrote in an analysis, “Mr. Corbyn’s far-left electoral message of increased social spending and higher taxes on the rich has resonated with the U.K.’s young and disaffected voters.”

But security and counterterrorism appeared to be dominating voter imagination as the polls opened. Rachel Sheard, a 22-year-old who cast her ballot near the site of Saturday’s attack in London, said the election had not gone as expected.

“They wanted this election to be very much a kind of Brexit vote, and I don’t think that’s in the hearts and minds of Londoners at the minute, [not] nearly as much as the security is,” said Ms. Sheard. “It was very scary on Saturday.”

As Mrs. May’s lead shrank in the campaign’s closing days, she turned to talking up the dangers of a Labor victory, warning that it could result in a “coalition of chaos.”

But many in Britain’s middle had concerns about Mrs. May, whose leadership image was undercut when Mr. Corbyn held his own in 90 minutes of questioning last week during their only head-to-head debate.

Expectations for Mr. Corbyn, 68, had been rock-bottom when the campaign began. An old-line leftist who, for example, gushed over Fidel Castro on the day the longtime Cuban dictator died, Mr. Corbyn was saddled with the image of a weak leader who couldn’t even command the support of many fellow Labor members in Parliament.

But his proposals to renationalize Britain’s aging rail network and scrap university tuition fees have won him kudos among the socialist party’s base.

Mr. Corbyn has hinted that he might seek an alliance with the Scottish National Party, which is pushing for another independence referendum for Scotland even though 55 percent of Scottish voters opted to remain in the United Kingdom just three years ago.

⦁ Benjamin Plackett contributed to this article, which is based in part on wire service reports.

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