May herself had said during her campaign: "It's a fact that if we lose just six seats, we will lose our majority and Jeremy Corbyn will become prime minister", predicting that the Scottish Nationalists and Liberal Democrats would back him.
The SNP has said it will not enter a formal coalition with Labour but could support them through confidence and supply.
Corbyn argued that the real danger comes from Conservative cuts to police budgets.
"We won't defeat terrorists by ripping up our basic rights and our democracy", he said.
Polls will be open Thursday from 7 a.m.to 10 p.m. (0600GMT to 2100GMT), with all 650 seats in the House of Commons up for grabs.
"Generally, on average, the polls tend to point toward a fairly small Tory majority, whereas the betting is more optimistic of the prime minister landing a majority of around 70 seats", said a betting expert at Ladbrokes, one of the UK's leading bookmakers.
Prime Minister Theresa May's Conservative Party will fail to win a parliamentary majority in Britain's election, according to an exit poll on Thursday, a shock result that would plunge domestic politics into turmoil and could delay Brexit talks.
The Times is the only one not to lead with the election, focusing instead on footage of the London Bridge attackers "laughing, joking and hugging as they plotted their murderous rampage".
Any delay in Brexit talks would reduce the time available for what are expected to be the most complex negotiations in post-World War Two European history. "There is a very clear choice for people when they come to vote".
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The deadly attacks in Manchester on May 22 and London on Saturday twice brought the campaign to a temporary halt - and put the threat from worldwide terrorism front and centre. He opposed British military interventions in Afghanistan, Iraq and Libya, wants to scrap Britain's nuclear arsenal and shared platforms with Irish republicans in the years when the IRA was setting off bombs in Britain.
Conservative-supporting newspapers went on the attack against Corbyn on Wednesday.
Predictions usually start appearing shortly after the polls close, although we know from recent elections that exit polls can be way off the mark. "They underestimated the good sense of ordinary people, ordinary people all over Britain".
May went into the election with a reputation for quiet competence, and campaigned on a promise of "strong and stable government".
But the Conservative government's record on cutting funding for health and education have also featured strongly in the campaign, to the benefit of Labour.
May's tightly controlled campaign style, which saw her speaking largely to hand-picked groups of supporters, began to grate. Arguably that would be an even more shocking result than Trump winning the presidency, especially after the UK's righward lurch in the Brexit referendum. When it resumed, May was facing questions about her record as Britain's interior minister between 2010 and 2016 - years in which police budgets were cut and the number of officers fell by nearly 20,000.
With security high, millions of voters are casting their ballots in polling stations ranging from schools and public buildings to churches, pubs and even a windmill and a launderette.
By contrast, Labour leader Jeremy Corbyn, a veteran socialist who had initially been written off as a no-hoper, was widely deemed to have run a strong, policy-rich campaign that enthused many followers. Usually you'd expect terror to nudge voters towards the right, where parties are more hawkish and trumpet more muscular security, but May was compromised by the fact that she was Home Secretary for years before becoming PM.
"We've had our fingers burnt by coalition, I don't need to tell you that".