LONDON — Millions of voters headed to the polls across the United Kingdom on Thursday for the country's third nationwide general election in less than five years.

Prime Minister Boris Johnson's Conservative Party is facing off against the opposition Labour Party, led by Jeremy Corbyn, for the keys to No. 10 Downing St.

Smaller parties include the Liberal Democrats, the Brexit Party, the Scottish National Party, Wales' Plaid Cymru, and the Green Party — any of which might be asked to form a coalition if no clear winner emerges from the big two.

Boris Johnson speaks to supporters at a factory in Manchester, England, on Tuesday. Christopher Furlong / Getty Images

The vote has been billed as potentially the most pivotal for the country in years. It has been a campaign dominated by Brexit — Britain's ongoing debate around its departure from the European Union — as well as the future of its publicly run National Health Service.

British broadcasting rules prevent discussion or analysis of election issues after the polls open at 7 a.m. local (2 a.m. ET). An exit poll produced by three broadcasters is published when voting closes at 10 p.m. local time, and has in recent years provided an accurate prediction of how the parties have fared.

Most results won't be confirmed until the early hours of Friday morning, with results from some tight races and votes in remote parts of the country delayed until much later that day.

How does the election work?

In the U.K. there are no separate races to choose the government and the legislature (like the presidential and congressional elections in the U.S). Here in the U.K. they are combined into one.

The country is split into 650 areas, known as constituencies. People in every constituency vote for one lawmaker they want to send to the House of Commons.

Any party that wins 326 seats or more would have a majority in the Commons and therefore becomes the new government. The victorious party leader usually becomes prime minister and selects his or her Cabinet members from their party's newly elected lawmakers.

Jeremy Corbyn at a campaign event in Stainton Village, England, on Wednesday. Oli Scarff / AFP - Getty Images

Things get complicated when no party achieves more than 50 percent of these parliamentary seats. That happened at the last election in 2017. Although Theresa May's Conservative Party won more seats than anyone else, it did not get more than half of the total.

That forced it to enter into an informal coalition with the Democratic Unionist Party.

The DUP, as its known, is one of the smaller parties that dominate the 18 seats in Northern Ireland, alongside Sinn Fein, the Social Democratic and Labour Party, Alliance, and the Ulster Unionist Party.

In Northern Ireland the mainstream British parties don't have much of a presence.