LONDON — The exit polls for Thursday’s UK general election show that Prime Minister Theresa May’s Conservative party, which controlled the government prior to the election, will lose 17 seats in Parliament — while the opposition Labour Party, and led by far-left firebrand Jeremy Corbyn, will gain 34 seats.

This is a big, big deal. If it’s right, it means the UK will have what’s called a “hung Parliament” once you factor in third parties. This means no party will control an outright majority of seats in Parliament (326 out of 650), which means it’s not obvious who the prime minister will be going forward. May is likely to keep control of the government — but Prime Minister Jeremy Corbyn can’t be ruled out.

Exit poll: HUNG PARLIAMENT

Tories - 314 (-17)

Labour - 266 (+34)

SNP - 34 (-22)

Lib Dems 14 (+6)

Plaid - 3

Green - 1

UKIP - 0 — Nick Eardley (@nickeardleybbc) June 8, 2017

The exit polls are generally considered quite accurate in the UK — unlike the preelection polls, which have a long history of mistakes. Data provided by the Financial Times shows that the exits have been really close for the past five elections. So while these results aren’t definitive — there could be some surprise error this year — there’s good reason to take them seriously.

And what they’re showing is a stunning Labour come-from-behind surge, throwing the entire UK political system into chaos in the midst of the Brexit negotiations and at a time of unprecedented tensions in the broader Western alliance pegged to President Donald Trump.

“This suggests that there will be some instability — and it will be hard for the UK government to negotiate a position on Brexit,” says Tony Travers, director of the London School of Economics’ Institute of Public Affairs.

Buckle up.

How a hung Parliament may have happened — and how it works

The deep irony of these results — if they are, in fact, right — is that there wasn’t supposed to be a British election until 2020. May called an early election on April 18, something PMs can do in parliamentary systems, because she had good reason to believe she’d win by a massive margin. Labour was down by about 16 points in the national polls; Corbyn had a net-negative approval rating among voters from his own party.

Since then the preelection polling had tightened, showing Corbyn behind by a little under 8 points in the national poll averages heading into today’s vote. This owed largely to May’s own screw-ups, most notably a callous health care proposal that would functionally tax elderly people for getting dementia. Corbyn, meanwhile, appeared to run a much better campaign that expected, seeming stable and assured compared to May. Other shocks during the campaign — most notably two terrorist attacks — didn’t appear to fundamentally alter the race.

The exit polls suggest that people were significantly underestimating May’s decline. The most likely reason why is that Corbyn’s support was concentrated among young voters and people who didn’t vote in the last election, groups that typically don’t vote in high numbers.

Moreover, it appears that the Scottish National Party — a pro-Scottish Independence Party that had sapped seats away from Labour in progressive Scotland — declined significantly. That would indicate a Labour surge in Scotland.

Now, we can’t be too precise on the numbers. Historically, British exit polls have been off by roughly 15 seats. Their estimate for the Conservative total, about 314, is within that margin — remember, a majority is 326. So if they’ve underestimated the Tories by 12 seats, which is possible, the country may be able to avoid a hung Parliament.

But if a hung Parliament or even close — a four seat majority isn’t much better — it’s chaos.

There are basically two options for how to deal with it. The first is that the Tories or Labour can form a coalition with one or more of the smaller parties, essentially pooling their votes to create a majority. The Tories, remaining the largest party, would get the first crack at making a coalition — but if they couldn’t form one, Labour would have a chance.

But given the wide difference in policy views among the small parties and between them and the major ones — they range from anti-immigrant demagogues to pro-Europe dead-enders to Scottish secessionists — it’s not clear how a coalition could be formed.

“I doubt it will be a coalition,” Travers says.

The other alternative is something called a minority government, where the Tories maintain control but don’t have enough votes to pass major legislation. This has happened only a handful of Times in UK politics: 1923, 1974, and 1977-78.

It usually doesn’t last long for a reason. Minority governments have to rely on votes from other parties to do anything, including agreeing to the terms of a Brexit agreement with the European Union (which the Tories have committed on doing).

Again, we shouldn’t assume any of this is going to happen. It’s possible that the exit poll completely blew it.

“There was one major error before, in 1992,” Patrick Dunleavy, a professor at the LSE, says. “But the reason that happened in 1992 is not likely to be the reason [for error] today.”

But if it’s even close to right, then it is a massive, massive defeat for Theresa May and the Tories — and a huge shock to Britain’s political system.