LONDON — An exit poll projection suggested Thursday that British Prime Minister Theresa May’s gamble in calling an early election has backfired spectacularly, with her Conservative Party in danger of losing its majority in Parliament.

An opposition Labour Party that had been written off by many pollsters surged in the final weeks of a campaign that was marred by deadly attacks in Manchester and London. If accurate, the result will confound those who said Labour’s left-wing leader, Jeremy Corbyn, was electorally toxic.

The survey predicted the Conservatives will get 314 seats and the Labour Party 266. It projected 34 for the Scottish National Party and 14 for the Liberal Democrats.

Based on interviews with voters leaving polling stations across the country, the poll is conducted for a consortium of U.K. broadcasters and regarded as a reliable, though not exact, indicator of the likely result.

If confirmed, the result will be humiliating for May, who called a snap election in the hope of increasing her majority and strengthening Britain’s hand in exit talks with the European Union. If she has failed, she could face pressure to resign.