LONDON — Britain could be heading for a hung parliament and days of political chaos as a shock exit poll predicted that Prime Minister Theresa May has fallen 12 seats short of an overall majority.

The poll, published as polling stations closed at 10 p.m. U.K. time, predicts the Conservatives will win 316 seats, down 17, with Jeremy Corbyn’s Labour party behind on 266, up 34. At around 2.30am local time, with around 150 seats declared out of 650 in total the BBC revised its forecast to 322 Conservative seats with Labour on 261.

With other parties picking up the remainder of the 650 parliamentary seats, the projected result would leave May’s Conservatives short of an overall majority and only able to govern with the support of other parties.

But with only the Northern Irish Democratic Unionist and Ulster Unionist parties, likely to win between 8 and 11 seats between them, likely to back her, such a result could open the door for Jeremy Corbyn to lead a minority Labour government with the support of the Scottish National Party (SNP), who are projected to take 34 seats, the Liberal Democrats, projected to win 14, the Greens, Welsh national party Plaid Cymru and the Northern Irish Social Democratic and Labour Party.

Labour were cheered by early results, taking Battersea in London from the Tories' junior treasury minister Jane Ellison, overturning a majority of nearly 8000. They also unseated the former Deputy Prime Minister Nick Clegg in Sheffield Hallam. Labour took the ultra-marginal seat of Vale of Clwyd from the Tories and the Rutherglen and Hamilton West seat from the SNP.

The Conservatives appear to be faring better in Scotland, where they came from third place to take the seat of Renfrewshire East from the SNP. They also knocked out the SNP's deputy leader Angus Robertson in Moray and took Angus, where the SNP's vote dropped 17 percentage points.

The Lib Dems were cheered by the result in Dunbartonshire East where former minister in the coalition government Jo Swinson took the seat.

Official results from the U.K.’s 650 constituencies will be announced through the night, with the final result likely to be clear by Friday morning. But the exit poll, compiled by Ipsos MORI and GFK for the BBC, ITV and Sky and based on interviews with voters outside 144 polling stations, has been an accurate or near-accurate prediction of the final result in recent general elections.

In 2015 the exit poll correctly predicted that David Cameron’s Conservatives had come first — despite previous polls putting the lead parties neck-and-neck — and in 2010 it correctly predicted a hung parliament.

The projection, if it is borne out, would confound the expectations of polling companies which, while varying, had all predicted a Conservative majority.

It would also be an extraordinary humiliation for May, who called the election seven weeks ago with the intention of increasing her slim working majority of 17 in parliament, and securing a personal mandate for her program of government, and her strategy for the U.K.’s Brexit negotiation with the EU, which is due to begin on June 19.

In the event of a hung parliament, May as the incumbent prime minister would have the first opportunity to form a government. If the projection is accurate, much will depend on the outcome in Northern Ireland. The only two parties likely to support a Tory government are the DUP and UUP. Republican party Sinn Fein, meanwhile, do not traditionally take their seats in parliament and their numbers could influence whether May can form a majority with the support of the DUP and the UUP, or whether Corbyn, with support of other parties, would command most support in parliament.

One of the first seats to declare, Newcastle upon Tyne Central, a Labour hold in a safe seat, saw a 10 percent gain for Labour, a six percent gain for the Conservatives, with UKIP dropping 11 percent and the Lib Dems one percent.

Polling expert John Curtice told the BBC: “It is a 2 percent swing to Labour, yes it is a safe seat, but it is the first sign of the night that maybe the country is going to drift from the Conservatives to the Labour party.”

Former Chancellor George Osborne, who Theresa May sacked on becoming Prime Minister last year, said on ITV that the Conservatives were facing a “completely catastrophic” set of results.

“It’s difficult to see, if these numbers were right, how they would put together the coalition to remain in office,” said Osborne, who now edits the Evening Standard newspaper.

A Labour campaign official said that, if the poll was accurate, it was fatal for May’s authority. If the poll is correct, the official said, it would be the biggest increase in a party’s share of the vote during a campaign. Labour began the campaign behind by double-digits in the polls. “We’re staying calm for now,” the official added.

“Let’s wait and see if it’s borne out” — Conservative campaign official

Defense Secretary Michael Fallon told the BBC, “This is a projection, it’s not a result. These exit polls have been wrong in the past. In 2015, they underestimated our vote, in a couple of elections before that, they overestimated our result.”

Conservative campaign officials are understood to be skeptical. One official said: “Let’s wait and see if it’s borne out.”

Labour’s Shadow Chancellor John McDonnell agreed that “some skepticism” was needed.

He told the BBC, “We tried to have an extremely positive campaign, we modeled it around Jeremy’s character…if it is reflected in this levels of support it does change the nature of political discourse in our country.”

Stewart Hosie, SNP candidate for Dundee East, deputy leader of the SNP Westminster group said the poll pointed potentially to a hung parliament and confirmed his party would not support a Tory government.

“I don’t recall us ever voting for a significant Tory policy in the past and it would be hard to see in the current climate with an austerity, cuts, hard Brexit party that we’d want to support them in any way in this future parliament,” he told the BBC.

Menzies Campbell, a former Liberal Democrat leader, said it was “equally impossible” that his party would form a pact with either the Conservatives or Labour, or take part in a formal "progressive alliance" with other parties opposed to the Tories.

“Progressive alliance implies a commitment to support the government which happens to be in power and the notion of a progressive alliance is that it would supplant the Conservatives.

“But, for example, on the issue of Brexit the Liberal Democrats’ position is very clear as compared to Jeremy Corbyn’s position which frankly almost defies definition. I can’t possibly see an arrangement of the kind between Labour and the Liberal Democrats which would in any way overcome that quite significant difference of opinion.”

However, he said that May had said she was willing to accept “the hardest possible Brexit” and this was not something that Lib Dem leader Tim Farron, who stood on a strongly pro-EU platform, could support.

The exit poll is encouraging for the Lib Dems who under this projection gain five seats. The SNP, however, fall from their 2015 peak of 56 to 34 seats.