LONDON — YouGov sent British politics into a frenzy on Tuesday evening when it published a new analysis suggesting that next month's general election could produce a hung parliament.
Prime Minister Theresa May announced a snap election in April and in the weeks that followed, the Conservatives led Labour by double figures in most opinion polls, with a landslide Tory victory looking like the only realistic outcome.
However, Labour has surged in recent polls, and YouGov's latest research suggests May could even end up losing a significant number of seats on June 8. This would surely be one of the most remarkable election results in British history.
But what actually is a "hung parliament"? And what would it mean in practice?
Here's everything you need to know.
In most previous British general elections one party has won an absolute majority in Parliament.
This is when one party wins more seats than all the other parties put together. Technically speaking, a party must win 326 of the 650 seats in the House of Commons in order to win an absolute majority.
However, in reality the winning threshold is 323, as MPs from Northern Irish party Sinn Fein don't attend parliament, as a matter of longstanding principle.
The First Past the Post voting system is designed to make this outcome more likely by translating smaller national leads in percentage terms, into bigger leads in terms of seats.
On rare occasions, the winning party does not gain enough seats to form an absolute majority, i.e. it has won fewer seats than all the other parties put together. This is "a hung parliament."
Britain has experienced just six hung parliaments since the beginning of the 20th century. The most recent occurred in 2010 when David Cameron's Conservatives won the election but with not enough seats to form an absolute majority.
Hung parliaments make passing legislation very difficult for the party in government. This is because, in theory, all other parties in the Commons can team up to defeat the government on laws it wants to pass.
This leaves the governing party with the following options:
1. Persuade another party or parties to support them with a so-called "confidence and supply" arrangement. Under this arrangement other parties would agree to support the government in any votes of confidence or budgetary votes, without which the government would fall.
2. Try to create a formal coalition by officially joining up with one or more party in the House of Commons to form a government with an absolute majority.
3. Go it alone. The winning party could decide to take a huge risk and try to run a government without an absolute majority. Parties that have tried this throughout British political history have not tended to last very long.
Cameron went with the second option in 2010. He persuaded the Liberal Democrats, then led by Nick Clegg, to join the Conservatives in a formal coalition, and give the Tories the numbers they needed to form an absolute majority.
If the election produces a hung parliament then May will likely have to go down the route of her predecessor Cameron and try to form a formal coalition government.
In theory, this could involve trying to strike a deal with the Liberal Democrats, assuming Tim Farron's party wins enough seats, or perhaps even the SNP, although the Tories and the SNP disagree on just about everything, not least Brexit and the question of Scottish independence. It may even mean May having to stitch together the support of several smaller parties in order to govern, the possibility of which she has previously dubbed a "coalition of chaos".
It's unlikely that May would try to go it alone as a minority government, given that many of her policies — the reintroduction of grammar schools, for example — face fierce opposition from MPs both from other parties and her own.
If May were to fail to form a coalition and didn't want to go it alone, then the only remaining option would be to hold another general election that would take place a few months down the line.
This situation happened in 1974, when a minority Labour government was elected in February of that year. Labour had the most seats but had taken fewer actual votes than the Conservatives. Prime Minister Harold Wilson called another general election in October 1974, and Labour won a three-seat majority. That government survived until 1979 but it was incredibly unstable. By-elections eliminated Labour's majority, and a pact with the former Liberal Party in 1977 eventually collapsed, leading to the 1979 election.
Today, a second election in the space of a few months would also be Britain's third election in just three years.
It's hard to imagine the public being enthused about yet another campaign, and the history of such second elections suggests that it would likely lead to a very similar result to the first.
The time required to both campaign for and hold a second general election would also eat up months of time that was supposed to be used for Brexit negotiations with the European Union.
A hung parliament could spell total chaos.