Voting began at some 41,000 polling stations across Britain on Thursday in a contentious and closely fought referendum on whether or not the country should leave the European Union.
About 46.5 million people are registered to vote, with experts predicting turnout of 60 per cent to 80 per cent. Rainstorms were expected to dampen turnout in London and other parts of southern England.
Voters are asked the single question: Should the United Kingdom remain a member of the European Union or leave the European Union?
A final poll by Opinium put the Leave vote on 45 per cent and Remain on 44 per cent, with 9 per cent undecided, as the online survey of more than 3,000 adults produced a "statistical dead heat."
"A brutal debate, dominated by personal attacks and misleading claims, has seen Leave gradually make ground on the once-dominant Remain campaign," The Telegraph newspaper commented late Wednesday.
But bookmakers and investors gave Remain a higher chance of victory than Leave, while an online survey of more than 5,000 people published Tuesday suggested that around one-third of voters had not made a final decision.
Prime Minister David Cameron, under pressure from those in his own Conservative party and a rising euroscepticism among British voters, promised in 2013 to hold a referendum on EU membership by 2017.
Cameron, who wants Britain to stay in the EU, called the referendum in February after negotiating a package of reforms that he said gave Britain a "special status" in the bloc.
Much of the debate focussed on the economy and EU migration, particularly in the last few weeks before the referendum.
"You don't tackle immigration by tanking the economy," Cameron told ITV between a flurry of events on Wednesday's final day of campaigning.
Polls are open until 10 pm (2100 GMT), with the first results from local counts expected in the early hours of Friday and the final result around 7 am Friday.