YouGov’s latest seat estimate predicts that neither party will win an overall majority, putting the Tories on 305 seats – 21 less than is needed, and Labour on 268.
If the YouGov projection is accurate, then Britain faces its first hung parliament since the 2010 election, which produced a Conservative/Liberal Democrat coalition.
Senior civil servants have reportedly been warned to prepare for an electoral stalemate, with sources saying that Labour frontbenchers have been in increased talks with Whitehall officials.
There are three possible outcomes in the event of a hung parliament.
The party with the most seats attempts to govern alone, without a majority.
Despite the narrowing in the polls, the Conservatives are still on course to win the most seats in the 2017 election and could form a minority government if they fail to win the required 326 seats for a majority.
However, minority governments are generally regarded as unstable and inefficient and Mrs May would be at the whim of opposition parties, relying on their support to vote through legislation.
Harold Wilson formed a minority Labour government in the February 1974 election.
He called another election in October that year, which saw him win a majority of three. However this was whittled away through by-elections, creating another minority administration.
John Major led the most recent minority government, after his 21-seat Conservative majority was worn away through defections and by-elections in 1996. A year later, Tony Blair’s New Labour won the general election.
Election 2017: Theresa May has warned of a 'coalition of chaos'
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Election 2017: Jeremy Corbyn could form a coalition government
Two or more parties enter into a formal coalition with a combined majority.
Mrs May has repeatedly warned that Britain faces a “coalition of chaos” if the Conservatives do not win an overall majority.
The latest YouGov seat estimate predicts that Labour would need the support of the SNP and the Liberal Democrats to form a coalition.
Mr Corbyn has said that is campaigning for a Labour majority, but last week said: “You’d better ask me [about a coalition] on June 9.”
Ms Sturgeon has said that she “doesn’t envisage any formal coalitions”. She has explicitly ruled out working with the Conservatives, although the two parties would likely have the parliamentary numbers necessary.
Tim Farron has ruled out working with either Mrs May or Mr Corbyn over their plans to take Britain out of the single market.
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Confidence and supply
A minority government is propped up by other parties but does not enter into a formal coalition.
In this scenario, the smaller party (or parties) agrees to back the government on the Budget and other key votes, but judges all other bills on merit.
Like a formal coalition, a confidence and supply deal would likely result in unstable government.
The Prime Minister would likely be forced to enact some of the smaller party’s policies in order to win support for his or her own manifesto pledges.
In the event of a hung parliament Labour and the SNP are the most likely options for such a deal.
Ms Strurgeon has said that while she opposes a formal coalition, she would be open to forming a "progressive alternative to a Conservative government".
She added that she would “look at that on an issue-by-issue basis”.
Jeremy Corbyn has avoided questions on a confidence and supply deal, instead saying that he hopes to win outright.
Jim Callaghan’s Labour Party signed a sort of confidence and supply deal with the Liberal Democrats after losing its majority in 1977.
Years later, Mr Callaghan said that he regretted the agreement, which he said had “doomed the Labour party”.