WASHINGTON — (CNN) 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.

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 divisive campaigning, a record number of people — almost 46.5 million — are registered to take part in the historic vote.

Britons headed to polling stations under 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 floodwaters inundated them.

Others across the capital opened late due to the weather.

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 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.

The Met Office, the UK’s national weather service, issued a warning Thursday, forecasting another wave of thunderstorms breaking out across the southeastern part of the country, including the London area.

As soon as the UK’s 41,000 polling stations opened at 7 a.m. (2 a.m. ET), British citizens aged 18 and over, along with Irish and Commonwealth citizens living in the UK, began delivering their verdict at the ballot box.

CNN crews reported brisk voting, and members of the public posted photos of busy polling stations across the country.

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

The question they’ve been pondering for months: Should the United Kingdom remain a member of the EU or leave it?

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

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.

European citizens living in the UK are not eligible to vote unless they’re from Malta or Cyprus — both members of the Commonwealth, while citizens of Ireland are eligible to vote if they live in the UK.

Members of Britain’s House of Lords are also eligible to vote — even if they cannot cast ballots in general elections.

Some schools across the UK are closed for the day, to serve as polling stations, but it is class as usual for others.

A polling station in Huddersfield in northern England was closed for half an hour Thursday evening after a man was stabbed and collapsed in a nearby street.

Police said the stabbing was not believed to be linked to the referendum issue and the station has since reopened.

Strict rules mean that broadcasters, such as CNN, are limited in what they can report while polls are open.

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).

The final, nationwide result is expected to be announced Friday morning.

Once the results are in, they will be announced by region rather than by constituency.

The 12 regions include Scotland, Northern Ireland, Wales, London and eight English regions.

The Electoral Commission tweeted that it was fine for voters to fill out their ballot papers in pencil or pen after social media users expressed concern they received pencils at polling stations, saying they feared their votes could be tampered with.

“(U)sing a pencil to vote… is fine. But you can use your own pen if you wish. Just don’t leave it behind!” the tweet says.