The BBC/Sky/ITV poll suggested the UK was heading for a hung parliament, with Conservatives 12 seats short of the 326 they need for an absolute majority in the Commons.
The poll put Tories on 314 seats, with Labour on 266, the Scottish National Party on 34, Liberal Democrats on 14, Plaid Cymru on three and Greens on one.
If borne out by the actual results, the poll figures would represent a humiliation for the Prime Minister, who went into the election with a small but viable majority and expectations that she should be able to secure an advantage of 100 seats or more in the House of Commons by going to the country early.
And it would be a personal triumph for Labour leader Jeremy Corbyn, who was widely regarded as having run a successful campaign after being written off as unelectable by many observers and some in his own party.
It would also represent a significant setback for the SNP’s Nicola Sturgeon, whose party won a historic 56 out of 59 seats north of the border just two years ago.
And it could throw the UK’s politics into disarray as the parties scrabble to form a government, just 11 days before the expected start of Brexit negotiations in Brussels.
The poll suggests the Tories will lose 16 of the 330 seats they held at the end of the last Parliament, while Labour gains 37, the SNP loses 20 and the Liberal Democrats gain five.
However, even after 30,000 voters were questioned at 144 polling stations, there is always a possibility that the exit polls may be misleading.
In 2015, they significantly underestimated the Tory tally, putting David Cameron’s party on 316 when it finally emerged with 331.
The exit poll triggered instant speculation over the shape of any coalition if no party has an overall majority in the Commons.
Even with the support of Northern Ireland unionists, Conservatives would struggle to form a viable administration without reaching out to other parties.
Meanwhile, a so-called “progressive alliance” bringing together Labour, Liberal Democrats, the SNP, Plaid Cymru and Greens would fall short of an absolute majority and produce a total only a few seats larger than the Tories on their own.
The one combination which would creep over the crucial 326 mark would be a repeat of the 2010 Tory-Lib Dem coalition, which has been explicitly ruled out by Lib Dem leader Tim Farron.
Labour, Lib Dems and the SNP have ruled out a formal coalition, speaking instead about the possibility of a minority administration being propped up on a vote-by-vote basis.
A Labour spokesman said: “If this poll turns out to be anywhere near accurate, it would be an extraordinary result. Labour would have come from a long way back to dash the hopes of a Tory landslide.
“There’s never been such a turnaround in a course of a campaign. It looks like the Tories have been punished for taking the British people for granted.”
A Lib Dem source said it was “too early” to comment on the exit poll, but indicated the party did not have significant ambitions for gains: “In this election holding our own is a good night.”
Newcastle Central beat Sunderland to become the first seat to declare at 11pm with Labour candidate Chi Onwurah winning.