Voting has closed in the Brexit referendum and, with polls suggesting the result remains too close to call, it looks like an uncomfortable night awaits Prime Minister David Cameron.
A Leave vote is widely expected to spell the end of Cameron's premiership after he spearheaded the Remain campaign.
Cameron will take some relief in the final poll - released just hours before voting stations closed their doors - which put the In side ahead by a margin of 52% to 48%.
But the Ipsos Mori survey for the London Evening Standard alone is unlikely to be enough to allay prime ministerial nerves, with other polls in the past 24 hours showing a lead for Leave.
With no exit polls being conducted by broadcasters, it is likely to be well into the early hours of Friday before a reliable picture emerges of how Britain has voted in what Cameron has described as the most important vote in a lifetime.
The final result is not expected to be confirmed at the Electoral Commission's main counting centre in Manchester until breakfast time.
A record number of voters are eligible to take part in the referendum, with the Electoral Commission putting the number at 46,499,537.
Anecdotal evidence from around the country suggests that turn-out has been high in many areas - something widely expected to favour the Remain camp. Fine weather in Scotland heartened Remain campaigners, who are relying on a high turnout north of the border, where voters are thought to be strongly pro-EU.
But torrential rain and flooding in the South East caused transport disruption which may have prevented some voters from reaching the ballot box in time. Some polling stations were forced to close, and two in Kingston-upon-Thames had to be relocated after becoming inundated.
Leave campaign standard-bearer Boris Johnson had a last-minute dash to vote in north London, due to a delay to his flight from Scotland after attending his daughter's university graduation ceremony - finally reaching the polling station with less than 25 minutes to spare.
Waiting for his plane in Edinburgh, he told reporters that polls had been "very close" but turnout was "good in areas where we need it to be".
Ukip leader Nigel Farage insisted his side could pull off an historic victory if "soft" Remain voters in the capital stayed home because of the bad weather. Those backing Brexit had the "real conviction and passion" to cast their vote regardless of the conditions, he said.
Prime Minister Cameron ignored questions as he arrived with wife Samantha to cast their votes at Methodist Hall in Westminster, saying only "good morning" to the gathered media from across the world.
Labour leader Jeremy Corbyn - who has been accused of campaigning half-heartedly in the Remain cause - said he was "extremely" confident of the outcome, pointing to bookmakers' odds which have consistently favoured continued EU membership despite fluctuations in the polls.
Pro-Leave Justice Secretary Michael Gove said he was feeling "quite excited" as he accompanied his wife, Sarah Vine, to a polling station in west London.
The referendum has seen one of the longest, and most personally bitter, campaigns in recent British political memory.
Both sides of the campaign have been locked in fierce fighting for months, and things came to a frenetic close on Wednesday as senior politicians criss-crossed across the country to try to sway undecided voters.
The Prime Minister and his Remain colleagues from across the political spectrum have warned of the potentially severe economic consequences of a Brexit vote amid fears of financial market turmoil and another recession.
But Leave campaigners, led by former London mayor Johnson, have urged voters to "take back control" of the country.
Meanwhile, dozens of celebrities have intervened during the course of the campaign to make their feelings known.
Footballer David Beckham, James Bond actor Daniel Craig and Harry Potter author JK Rowling were just three of the high-profile names to back the Remain campaign, while Leave won support from the likes of comedian John Cleese, former cricketer Sir Ian Botham and former England football player Sol Campbell.