What Is a Hung Parliament? U.K. Elections Exit Polls Predicts May to Lose Majority, So What Happens Next?

A supporter waits for Jeremy Corbyn, leader of Britain's opposition Labour Party, to arrive at a campaign event in Reading on May 31. A shock projection of results suggests Prime Minister Theresa May will fall short of the majority needed to form a government. Peter Nicholls/Reuters

The U.K. election was supposed to be a foregone conclusion. Virtually everyone working in British politics thought Conservative Prime Minister Theresa May was certain to win.

But a shock projection of the results, released at 10 p.m. local time on Thursday immediately after voting ended, showed May would win just 314 of the British parliament's 650 seats. The figure represents a drop from her current 330 and would put her short of the majority needed to guarantee the chance to form a government.

The poll showed that Jeremy Corbyn, leader of the left-wing Labour Party, would win 266 seats; the centrist Liberal Democrats would win 14; hard-right UKIP would win zero; the environmentalist Green Party would win one; and the Scottish National Party (SNP) would win 34.

This scenario, in which no party has a majority, is called a hung parliament. If the poll turns out to be true—and it isn't definitive—it means Westminster is about to enter a period of frantic negotiations.

In short: There's no clear prediction of who will form Britain's next government.

The exact number of seats required for a working majority in Britain depends on how well Sinn Fein, a Northern Irish party, does in the election. Its MPs, radical Irish nationalists, do not take seats in the British parliament, so the more of them there are, the fewer seats a party parliament effectively needs to win a majority.

That means the number for a majority could be anywhere between 318 and 326.

If no party wins a majority by itself, it can try to form a government in one of two ways. Either it can form a coalition with one or more other parties, or it can form a minority government, where it has no majority by itself but relies on other parties to make deals on a vote-by-vote basis to pass legislation.

At this stage, politicians are being very cautious, insisting that the poll may not be accurate. But we do have some clues about which parties might work with each other.

The Scottish National Party (SNP), which supports Scottish independence and is socially and economically left of center, has intimated it would be happy to work with Corbyn's Labour.

The Liberal Democrats, meanwhile, have said they will not work with either main party. They entered a coalition with the Conservatives from 2010 to 2015 but were badly burned by the experience, which damaged the party's reputation among voters. But they made that pledge when everyone expected a Conservative win; it will be interesting to see if they keep it.

The exit poll also shows one seat for the environmentalist Green Party and three for the left-of-center Welsh nationalist party Plaid Cymru, both potential partners for Labour.

The poll shows 18 seats divided among other parties including Sinn Fein and the Irish unionist party DUP, which is a natural partner for the Conservatives.

If the seat numbers prove to be accurate, it would be impossible for naturally allied parties to form a majority if Liberal Democrats stick to their promise not to enter the coalition. Even with the SNP and the Liberal Democrats, the Labour Party would still be short of a majority.

The actual results, expected throughout the night and into Friday morning, might change the picture. But at this stage, it looks like Westminster and the United Kingdom are in for a period of serious uncertainty.