With no clear winner likely to emerge from Thursday's vote, a wounded May vowed to provide stability, while her Labour rival Jeremy Corbyn said she should step down.
An updated BBC forecast predicted May's Conservatives would win 318 of the 650 House of Commons seats, eight short of a majority, while the left-wing opposition Labour Party would take 267 -- producing a "hung parliament" and potential deadlock.
Sky News also predicted May would lose her majority, scoring somewhere between 315 and 325 seats.
With talks of unprecedented complexity on Britain's departure from the European Union due to start in just 10 days' time, there was uncertainty over who would form the next government and over the fundamental direction Brexit would take.
"At this time, more than anything else this country needs a period of stability," a grim-faced May said after winning her own parliamentary seat of Maidenhead in southeast England.
"If ... the Conservative Party has won the most seats and probably the most votes then it will be incumbent on us to ensure that we have that period of stability and that is exactly what we will do."
After winning his own seat in north London, Corbyn said May's attempt to win a bigger mandate had backfired.
"The mandate she's got is lost Conservative seats, lost votes, lost support and lost confidence," he said.
"I would have thought that's enough to go, actually, and make way for a government that will be truly representative of all of the people of this country."
May had unexpectedly called the snap election seven weeks ago, confident of sharply increasing the slim majority she had inherited from predecessor David Cameron before launching into the Brexit talks.
Instead, she risked an ignominious exit after just 11 months at Number 10 Downing Street, which would be the shortest tenure of any prime minister for almost a century.
"Whatever happens, Theresa May is toast," said Nigel Farage, former leader of the anti-EU party UKIP.
May had spent the campaign denouncing Corbyn as the weak leader of a spendthrift party that would crash Britain's economy and flounder in Brexit talks, while she would provide "strong and stable leadership" to clinch a good deal for Britain.
But her campaign unravelled after a major policy u-turn on care for the elderly, while Corbyn's old-school socialist platform and more impassioned campaigning style won wider support than anyone had foreseen.
Sterling fell by more than two cents against the U.S. dollar after an exit poll showed May losing her majority.
SCOTTISH NATIONALISTS RETREAT
In Scotland, the pro-independence Scottish National Party suffered major setbacks. Having won all but three of Scotland's 59 seats in the British parliament in 2015, their share of the vote fell sharply and they lost seats to the Conservatives, Labour and the Liberal Democrats.
The SNP's leader in the British parliament, Angus Robertson, was among those who lost their seats.
The centre-left, pro-EU Lib Dems were having a mixed night. Their former leader, Nick Clegg, who was deputy prime minister from 2010 to 2015, lost his seat. But former business minister Vince Cable won his back, and party leader Tim Farron held on.
With the smaller parties more closely aligned with Labour than with the Conservatives, the prospect of Corbyn becoming prime minister no longer seems fanciful.
That would make the course of Brexit even harder to predict. During his three decades on Labour's leftist fringe, Corbyn consistently opposed European integration and denounced the EU as a corporate, capitalist body.
As party leader, he unenthusiastically campaigned for Britain to remain in the bloc, but has said that Labour would deliver Brexit if in power. The party has not given a detailed plan for Brexit but has said its priorities would be to maintain the benefits of both the EU single market and its customs union.
"As a Brexiteer who believes in it with all his heart and soul, my fear is that Corbyn forms a coalition with the SNP and a few Lib Dems and we look down the barrels of a second referendum in a few years' time," said Farage.
In domestic policy, Labour proposes raising taxes for the richest 5 percent of Britons, scrapping university tuition fees, investing 250 billion pounds ($315 billion) in infrastructure plans and re-nationalising the railways and postal service.
Early results were in line with the exit poll, with Labour doing better than expected. That was in part because votes that had previously gone to UKIP were splitting evenly between the two major parties instead of going overwhelmingly to the Conservatives as pundits had expected.
"UKIP voters wanted Brexit but they also want change," Farage said.
"They are fundamentally anti-establishment in their attitudes and the vicar's daughter (May) is very pro-establishment. And I think she came across in the campaign as not only as wooden and robotic but actually pretty insincere."
(Additional reporting by Guy Faulconbridge, Alistair Smout, David Milliken, Paul Sandle, William Schomberg, Andy Bruce, William James, Michael Urquhart and Paddy Graham in London, Padraic Halpin in Dublin; Writing by Estelle Shirbon; Editing by Mark Trevelyan and Mark John)
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