LONDON — U.K. voters were deciding Thursday who they want to resolve the stalemate over Brexit in a parliamentary election seen as one of the most important since the end of World War II.

Voting was underway across the country in a contest that pits Prime Minister Boris Johnson, who says he will take Britain out of the European Union by Jan. 31, against opposition leader Jeremy Corbyn, who promises another referendum on Brexit.

Johnson voted at Methodist Central Hall in London, accompanied by his dog, Dilyn.

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With so much at stake, political parties have pushed the boundaries of truth, transparency and reality during five weeks of campaigning.

Johnson's Conservative Party was criticized for using misleading tactics on social media, while Corbyn's Labour Party sought to win votes by promising to tax the rich, boost government spending and nationalize industries such as railroads and water companies. One of the focal points of the ugly campaign was the National Health Service, a deeply respected institution that has struggled to meet rising demand after nine years of austerity under Conservative-led governments.

Jill Rutter, program director for the Institute for Government, said one of the things that stood out during the campaign was the shamelessness of the politicians. She cited Johnson's claim that the Conservatives would build 40 hospitals. In fact that number includes many existing facilities that will be renovated.

“Normally, if you point out to people that something doesn’t stand up, it’s actually sort of fiction, you slightly expect them to start ... replacing that with a different new fact,'' Rutter said. “But here, actually, you’ve seen this from No. 10 under Johnson that they’re prepared to run a deeply manipulative operation.’’

All 650 seats in the House of Commons are up for grabs in the election, which is being held more than two years ahead of schedule.

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The prime minister called early elections in hopes of breaking a logjam in Parliament that stalled approval of his Brexit agreement in October. Johnson didn’t have a majority in the last Parliament and was stymied once he lost the support of the Democratic Unionist Party because of concerns about how Northern Ireland would be treated under his deal with the EU.

Opinion polls have consistently showed Johnson's Conservative Party in the lead, but recent surveys suggest the margin may have narrowed in the final days of campaigning. While Corbyn’s Labour is unlikely to win an outright majority, smaller opposition parties hope to win enough seats so they can team with Labour to block Johnson’s Brexit plans.

All of the parties are nervous about the verdict of voters who are more willing to abandon long-held party loyalties after three years of wrangling over Brexit.

The Conservatives have focused much of their energy on trying to win in a “red wall” of working-class towns in central and northern England that have elected Labour lawmakers for decades, but also voted strongly in 2016 to leave the EU. Polls suggest that plan may be working, and the Conservatives have also been helped by the Brexit Party led by Nigel Farage, which decided at the last minute not to contest 317 Conservative-held seats to avoid splitting the pro-Brexit vote.

Labour, which is largely but ambiguously pro-EU, faces competition for anti-Brexit voters from the centrist Liberal Democrats, Scottish and Welsh nationalist parties, and the Greens.

One of the campaign's defining images was a photo of a sick 4-year-old boy sleeping on a hospital floor because no beds were available. Johnson's initial failure to even look at the photo in an on-camera interview put the prime minister on the defensive because he was seen as being insensitive to the child's plight.

The photo, initially published by the Yorkshire Evening Post, swept across social media like a firestorm, injecting an explosive jolt into the political war of information in the final days of the election.

Social media platforms were a critical battleground during the campaign, with political mudslingers waging cyberwar with few legal constraints after the government failed to act on calls for a new law to protect democracy in the internet age. Just two years after Britain found itself at the epicenter of a global scandal over the misuse of Facebook data by political campaigns, the parties bombarded voters with social media messages — many of which were misleading.

The Conservative Party circulated a doctored video that made it look as if an opposition leader had been stumped when asked about his position on Brexit. Then during a television debate the party re-branded its press office Twitter account as a fact-checking service. The Labour Party also sought to co-opt the roll of independent fact-checker, rolling out a website called The Insider, which called on voters to “trust the facts.”

The Conservative Party, with a dynamic online campaign, found itself in the crosshairs of many media critics. Some, such as Rasmus Kleis Nielsen, director of the Reuters Institute for the Study of Journalism at the University of Oxford, wondered why Johnson, who has been in the lead, would feel compelled to be at the forefront of pushing the edge of accepted norms.

“This is a governing party. It's a mainstream party. It's a career politician. This is not an outlier. You don't get more establishment than the British Conservative Party," Nielsen said. “If that is what they see as fit and proper, we must confront the fact that this is the new normal.''

Matthew Goodwin, a visiting senior fellow at the Chatham House think tank, said the Conservative Party's tactics were partly motivated by alarm over the potential for a Corbyn-led government.

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“We have to remember this is probably the most consequential election we’ve had in the post-war period,'' he said, citing Brexit and other implications of the vote.

“For the Conservatives, the reason they have become so direct and so personal is, I think, they view this as paving the way for a Marxist project, a radical left-wing project that is more of a threat to this country, to the union, to Britain’s economy, to its place on the international stage — a greater threat than anything the country has faced before," Goodwin said.