Counting is well under way in the British referendum on membership of the European Union, with the lead switching back and forth between the Leave and Remain camps.
Gibraltar was the first constituency to return a result, with the British Overseas Territory voting overwhelmingly to Remain in the EU.
Newcastle, the Orkney Islands and Clackmannanshire also voted to Remain, but Leave won a significant majority - more than 30,000 votes - in Sunderland to move ahead overall by 50.5% to 49.5%.
Sterling took a massive drop on the global markets following the Sunderland result, falling to as low as $1.435 from a year high of $1.50 after the polls closed.
Subsequent results from Foyle and the Isles of Scilly swung the momentum back to the Remain camp before Swindon and Broxbourne voted to Leave.
UKIP leader Nigel Farage, who campaigned on the Leave side of the referendum, has also 'unconceded' the referendum.
Speaking on Sky News after 10pm, Mr Farage said it "looks like Remain will edge it".
However, a spokesman for Leave.eu says Mr Farage told him he does not have a better idea than anyone else about the result.
A YouGov opinion poll for Sky News after polls closed at 10pm indicated that Remain is on 52% and Leave is on 48%.
A subsequent Ipsos Mori survey which was taken Wednesday and Thursday and released after polls closed show Remain on 54% and Leave on 46%.
However, other polls released in the last 24 hours have put Leave ahead.
As a result, Prime Minister David Cameron faces a tense night as votes are counted across the UK, with a Leave vote widely expected to spell the end of his premiership, although a number of pro-Brexit Conservatives, including Boris Johnson and Michael Gove, have written to Mr Cameron urging him to stay on regardless of the result.
The letter, signed by 84 Tories, was delivered after the polls closed.
Mr Cameron called the referendum under pressure from his ruling Conservative Party and an increasingly powerful anti-EU party, hoping to put to rest decades of debate over Britain's place in Europe and its ties with Brussels.
Depending on turnout and on the margin by which the winning side prevails a clear indication is likely by the early hours of Friday.
The official declaration is due to be made in Manchester early Friday morning.
Mr Cameron is scheduled to make a statement outside Downing Street at 8am.
The campaign was dominated by immigration and the economy and shaken by the murder of a pro-EU MP.
Sterling rose against the dollar, reaching a six-month high, as traders continued to bet on the UK voting to remain in the EU.
Betting markets suggest a probability of over 80% that voters would opt to remain, but pollsters are still saying it is a very close race - too close to call.
The Leave campaign says Britain's economy would benefit from a Brexit, or British exit. Mr Cameron says it would cause financial chaos.
Traders, investors and companies are preparing for volatility on financial markets whatever the outcome of a vote that both reflects, and has fuelled, an anti-establishment mood also seen in the United States and elsewhere in Europe.
Much will depend on turnout, with younger people seen as more supportive of the European Union than their elders but less likely to vote.
A guide to Brexit | Brexit in quotes | Polling day in pictures
Polling stations opened at 7am and closed at 10pm, with counting beginning as soon as polls close.
There are 382 local counts, 12 regional count centres and one main count centre in Manchester from where the official declaration will be made.
It has been billed as the most important vote in a generation.
It's EU referendum day in the UK: here are today's UK front pages in one tweet (via @philipbromwell) #EUref #Brexit pic.twitter.com/adZufeOtaK
— RTÉ (@rte) June 23, 2016
Result from Northern Ireland expected around 4am
There will be a count for each of Northern Ireland'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 around 4am.
Sinn Féin, the Ulster Unionists, the SDLP and Alliance Party campaigned for a Remain vote in the referendum campaign while the Democratic Unionists, Traditional Unionist Voice and Ukip were among those advocating a Brexit.
The future of the Irish 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.
Meanwhile, the Cabinet will hold an early-morning meeting tomorrow if Britain votes to leave.
Minister for Foreign Affairs and Trade Charlie Flanagan has also said Ireland would have to consider taking steps to assist firms exporting into Britain if the Leave side wins.
