Polls closed Thursday in Britain's historic vote on whether to remain in the European Union or leave it. As many as 46.5 million paper ballots are now being counted by hand across England, Scotland, Wales, Northern Ireland and Gibraltar, a British overseas territory.

Results will be collected in 382 local authorities, then sent to 12 regional counting centers for announcement of the results by region. The process will take hours -- and could result in a country leaving the European Union for the first time ever.

[Previous update, posted at 3:25 p.m. ET]

British voters have just hours left to cast their ballots in Thursday's once-in-a-generation referendum on whether the United Kingdom should leave the European Union.

Following months of heated campaigning, a record number of people -- almost 46.5 million -- are registered to take part in the historic vote.

Happiest I've ever been to see a rainy queue at 7:30 in the morning #eureferendum #ivoted #CNNiReport A photo posted by Sophie Carr (@sophie_117) on Jun 23, 2016 at 12:53am PDT

With the country on edge about the vote, Britons headed to polling stations beneath rainy skies in London, following torrential rains and thunderstorms overnight that caused flash flooding in parts of the capital and southeastern England.

The downpour wreaked havoc on transport networks in London and southern England and caused two polling stations in southwest London to close and relocate after they were inundated with floodwaters. Others across the capital opened late due to the weather.

A flooded street in Battersea, London.

Some social media users complained they had to brave flooded streets and negotiate disrupted public transport networks to cast their ballots. Travel on parts of London's rail network was suspended due to flooding, and severely delayed on others, while train travel elsewhere across the south was disrupted.

Britain's Met Office issued an amber warning on Thursday, forecasting another wave of thunderstorms across the southeastern portion of the country, including the London area.

Britain's Electoral Commission tweeted that those delayed on the way to polling stations should know that as long as they were standing in line to vote at 10 p.m., they would be able to cast a ballot.

Registered voters include Britons aged over 18 from England , Wales, Scotland, Northern Ireland and Gibraltar -- a British territory off the southern coast of Spain. Irish and Commonwealth citizens living in the UK are also eligible to vote.

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Among the key political players casting their votes Thursday were UK Prime Minister David Cameron, who voted at a hall in London; Labour leader Jeremy Corbyn, who also voted in the capital; and UK Independence Party leader Nigel Farage, a leading "Leave" advocate, who voted in Westerham, England.

To accommodate the record numbers, some schools shut down to serve as polling stations, but it was class as usual for others.

A polling station in Huddersfield, northern England, was closed for half an hour Thursday evening after a man was stabbed and collapsed in the street nearby. Police said the incident was not believed to be linked to the referendum issue and the station has since reopened.

British PM David Cameron and his wife Samantha after casting their votes.

'Serious consequences'

Britain enters the referendum day a nation divided, ahead of a decision that will shape its place in the world for decades.

Polls have consistently shown voters split down the middle, with the outcome too close to call, and wavering voters likely to determine the result.

Leading political parties and newspapers are similarly divided on the so-called Brexit, or British exit, from the European Union -- an outcome that would be a huge blow to the 28-member bloc.

French President François Hollande warned Wednesday that the result would have a huge impact on the future of the European Union.

"The departure of a country that is, geographically, historically, politically in the European Union would have extremely serious consequences," he said.

The UK has been a member of the European Union (and its precursors) since 1973.

Retired soldiers prepare to vote on the EU referendum Thursday at London's Royal Hospital Chelsea.

Clashing sides

With many of the fundamental questions of the debate in dispute by the opposing "Leave" and "Remain" camps, the outcome may come down to a question of gut instinct for voters.

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The rival camps have clashed on such core issues as whether a Brexit would help or hurt Britain's economy, throughout a rancorous campaign that has repeatedly returned to themes of security, sovereignty and immigration.

"Leave" campaigner Boris Johnson, a member of parliament and former London mayor, described the EU during a final centerpiece debate Tuesday as "a job-destroying engine."

"You can see it all across southern Europe and you can see it, alas, in this country as well," he said.

Pro-"Leave" MPs Boris Johnson, Gisela Stuart and Andrea Leadsom take part in Tuesday night's debate.

By contrast, the "Remain" camp, led by Cameron, has argued that a vote to leave would do lasting harm to Britain's economy.

The "Leave" camp has framed the referendum as a historic opportunity to "take back control" of Britain from Brussels, where most of the EU institutions are headquartered, with Johnson describing it as a potential "independence day" for Britons.

Their rivals have argued that Britain will be safer, more powerful internationally and more prosperous if it remains in the bloc.

The "Leave" campaign has raked in more funds than its opponents, according to Britain's Electoral Commission.

The pro-Brexit camp received just under £15.6 million ($22.9 million) in donations, while "Remain" got £11.9 million ($17.5 million) from February 1 to June 9, it said.

'Out is out'

In addition to the massive changes that a vote for a Brexit would entail for Britain, observers predict it could also trigger a political crisis, with "Remain" supporter Cameron struggling to control his ruling Conservative Party if he backed the losing side on such a critical issue.

Cameron negotiated with European leaders this year to secure improved terms of membership in the bloc if Britain stays in the EU.

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European Commission President Jean-Claude Juncker warned Wednesday there will be no further renegotiation.

"We have concluded the deal with the Prime Minister; he got the maximum he could receive, and we gave the maximum we could give so there will be no kind of renegotiation," he said.

"Out is out."

Toxic climate

The political climate leading up to the referendum has been unusually volatile, with both sides accusing each other of lying and scaremongering.

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The tension reached its peak with last week's killing of Labour MP Jo Cox , a pro-"Remain" advocate in her first term in parliament. She was the first sitting British lawmaker to be killed since 1990.

Her husband, Brendan Cox, told the BBC she had been concerned about politics becoming "too tribal and unthinking."

"She was very worried that the language was coarsening and people were driven to take more extreme positions," he said.

On Wednesday, another Labour MP tweeted that she had received a death threat for her referendum campaigning.

Got this today for speaking out for Remain..... This has to stop. pic.twitter.com/HPV8asrZ3w — Yvette Cooper (@YvetteCooperMP) June 21, 2016

Final results

Polling stations across the UK will close at 10 p.m. (5 p.m. ET), with the first results expected about midnight (7 p.m. ET).

Voters line up in the rain to cast their votes in West Hampstead, London.

British citizens living abroad have already cast their votes by mail.

The final results are expected to be announced Friday morning.