The setting: A clear division

The UK government has been in the hands of either the right-of-center Conservative Party or the left-of-center Labour Party for a century. In more recent times, the Conservatives were in power from 1979 to 1997, Labour from 1997 to 2010 and the Conservatives since then, if you include the coalition government they led with the Liberal Democrats as junior members.

It is the same two parties battling for supremacy in this election, too. So the only two party leaders with a realistic chance of being prime minister after all the votes are counted are Conservative leader Boris Johnson and Labour leader Jeremy Corbyn.

There was a time not long ago when the most common complaint at election time in the UK was that political parties "were all the same." That is no longer the case, with Thursday's election providing the biggest difference between the parties that the country has seen since 1983.

The men who would be prime minister

Boris Johnson, 55, was educated at the same elite fee-paying school, Eton, that British royals Prince William and Prince Harry attended, before studying at Oxford University. A colorful character, he had a career in the media before becoming an MP and then being elected as Mayor of London. He was the figurehead of the pro-Brexit side in the 2016 referendum on whether the UK should stay in the European Union.

Corbyn, 70, went to a selective grammar school but decided that university was not for him. After time as a union official and Labour councilor he became an MP in 1983. He spent decades as a marginal figure on the backbenches, a left-winger at odds with the centrist policies that won Tony Blair three elections. He rebelled hundreds of times. He became leader in 2015, thanks in part to an influx of left-wing members, but against the wishes of the vast majority of his own MPs.

Some key policy dividing lines

Brexit: The Conservatives are campaigning to "get Brexit done." Johnson says the UK deal to leave the European Union will be completed by 31 January. Labour says it will scrap the current Brexit deal and instead negotiate a fresh one, which focuses on being good for jobs, before putting its deal to a referendum within six months in which people can either back it or vote to stop Brexit happening at all. 

The economy: The Conservatives are the continuity party when it comes to the economy. It wants to keep taxes low, but is also pledging to end the years of austerity by investing more money in schools, housing and hospitals. Labour, by contrast, has a huge sweep of changes planned – much larger spending plans for schools, housing and hospitals, and nationalizing the postal service, water, energy supplies, train operators and broadband provision. Business taxes, and personal taxes for the highest earners, are also going to be raised.

Foreign policy: The Conservatives back Nato, want to leave the EU and are, in general terms, keen on keeping close relations with the US. The Labour Party, whose leader used to lead the anti-war movement, says it will "not outsource" foreign policy to US President Donald Trump and will end a "bomb first, talk later" approach to security. It will invest more in diplomacy and legislate to stop a prime minister being able to take the UK to war without MPs backing it in a vote. 

What about other parties?

The Conservatives and Labour are the only two parties that can realistically win the UK-wide election, but in Scotland, the Scottish National Party, which is anti-Brexit and wants a fresh Scottish independence referendum, is expected to win convincingly. The third largest UK-wide party is the Liberal Democrats. A generally centrist party, its most eye-catching policy has been a pledge to cancel Brexit without a fresh referendum, if elected to power. 

The Green Party, which has one MP, and the Brexit Party which has none, will find it difficult to pick up more seats because of the first-past-the-post system used for British elections.

What is the current state of the parties?

What do the opinion polls say?

YouGov's poll is the largest to come out ahead of the election. Based on interviews with more than 100,000 people, it puts the Conservative lead at 9 percent – 43 percent to Labour's 34 percent.

YouGov models a national result based on its survey and is suggesting the Conservatives are set to win a majority in Parliament of around 28 seats, down from the 68 it predicted in a similar survey last month. The pollsters say its seat share predictions come with a margin of error, which means there is also a chance of the Conservatives falling short of a majority.

For the other parties, YouGov put the Liberal Democrats on 12 percent, with 15 seats; the Greens with 3 percent and one seat and the Brexit Party on 3 percent, with no seats. The SNP is projected to win 41 seats, an increase of six on 2017. While the outlook for Plaid Cymru, which only stands in Wales, is unchanged with four seats.

_{Polling expert John Curtice gives his assessment of the polls}

How does the election work?

The UK is divided into 650 areas, called constituencies, which each elect one Member of Parliament, known as an MP. The MP chosen is the candidate in that area who gets the most votes. 

The 650 MPs elected are normally from one of a small number of political parties, although occasionally a local independent candidate might get elected.

The UK has a parliamentary system rather than a presidential system, so it is up to MPs who have been elected to decide who the prime minister is, rather than people voting directly for the prime minister.

Read more:

A guide to election jargon

The election and UK foreign policy

Reviewing the parties' election campaigns

How important is climate change in the election?

YouGov poll refuses to rule out a hung parliament

Comment: Dirty politics intensifies as UK election looms

Five things: The Conservative manifesto

Five things: The Labour manifesto

Five things: The Scottish National Party manifesto

Five things: The Lib Dem manifesto