EU referendum results live: Nigel Farage 'concedes defeat' in Brexit vote
Leading Leave campaigner Nigel Farage appeared to be preparing to concede defeat with counting under way in the historic referendum on UK membership of the European Union.
The Ukip leader told Sky News that the Remain camp seemed likely to "edge it", but insisted that his party would continue its fight to take Britain out of the EU.
There were no exit polls in the vote, but a YouGov opinion poll released as polling stations closed put Remain ahead by 52% to 48%.
Mr Farage said: "It's been an extraordinary referendum campaign, turnout looks to be exceptionally high and (it) looks like Remain will edge it.
"Ukip and I are going nowhere and the party will only continue to grow stronger in the future."
Senior Leave campaigner Iain Duncan Smith cast doubt on Mr Farage's suggestion that Remain is set for victory.
"I never quite follow what Nigel Farage says," the former British work and pensions secretary told the BBC. "Quite often he says two different things at the same time.
"I genuinely do not have a sense of how this has gone."
It is likely to be well into the early hours of Friday before a reliable picture emerges of how Britain has voted in what Mr Cameron has described as the most important poll in a lifetime. And 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. In Gibraltar, which is taking part in the referendum as a British overseas territory within the EU, turnout was a healthy 84%.
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".
Earlier First Minister Arlene Foster, a Brexit campaigner, said it was important that those on both sides of the debate made their voices heard.
"I understand there has been a good turn-out in some of the areas where previously there hasn't been so far - so that's good to hear," she said.
The DUP leader added: "Everyone has the opportunity to cast their vote today and I hope they take that opportunity, regardless of what that might be, because this is a huge opportunity for all the citizens of the United Kingdom to make their voices heard in terms of the European Union."
Sinn Fein president Gerry Adams, a Remain advocate, also encouraged people to get out and vote.
"Come out and vote, if you haven't voted there is still time, vote to remain - it's not in Ireland's interests to do anything else," he said.
Polling stations opened at 7am and will close at 10pm. The count will begin shortly after the 600-plus stations close.
There will be a count for each of the region's 18 parliamentary constituencies, with those separate outcomes added together for an overall Northern Ireland result.
Counting will be conducted in eight centres across the region, with the main result expected to be announced at the Belfast count at about 4am.
Sinn Fein, the UUP, SDLP and Alliance Party campaigned for a Remain vote in the referendum campaign while the DUP, TUV and Ukip were among those advocating a Brexit.
The future of the border was a key issue in the campaign, with claim and counter claim on whether a UK exit would see a return to security and customs checkpoints.