Sterling plunged on overnight markets as traders responded to growing expectations that the UK may have voted to quit the European Union after 43 years.
Shadow chancellor John McDonnell said the Bank of England may have to intervene to shore up the currency, which lost 3% within moments of the first result showing a strong result for Leave in Sunderland and fell as much as 6.5% against the euro.
And Conservative backbencher Jacob Rees-Mogg said a general election in the autumn was “not impossible” as questions grew about the sustainability of David Cameron’s position as Prime Minister if voters reject his passionately-expressed advice to stick with the EU.
Strong performances for Leave in the English shires, Wales and north-eastern places like Sunderland, Middlesbrough and Hartlepool were countered by emphatic majorities in London for Remain, which also picked up large swathes of Scotland and Northern Ireland.
The Remain side was undermined by poor results in many of Labour’s traditional strongholds in the North, including Sheffield, where Mr Farage hailed a narrow victory for Leave as “amazing”.
The Ukip leader won ecstatic applause from Leave supporters as he told them at around 4am: “I now dare to dream that the dawn is coming up on an independent United Kingdom.”
Mr Farage told cheering supporters at a Leave.EU campaign party in London that June 23 should go down in history as ” our independence day”. He later said David Cameron should resign “immediately” as Prime Minister if Brexit won what was still expected to be a knife-edge vote.
“If the predictions now are right, this will be a victory for real people, a victory for ordinary people, a victory for decent people,” said the Ukip leader.
“We fought against the multinationals, we fought against the big merchant banks, we fought against big politics, we fought against lies, corruption and deceit, and today, honesty, decency and belief in nation I think now is going to win.”
Mr Farage said he hoped victory for Brexit would “bring down this failed project” of the European Union.
Pro-Leave Tories including Boris Johnson and Michael Gove signed a letter to Mr Cameron urging him to stay on as leader whatever the result of the referendum.
But others in the party raised questions over Mr Cameron’s future. Influential backbencher David Davis said the Prime Minister could stay on for a “couple of years” but should put someone else in charge of negotiations on a new relationship with the EU.
And long-standing Eurosceptic John Redwood suggested Tories should wait to see if Mr Cameron was willing to “implement the public will” after a Leave vote. Mr Redwood said the PM should bring in talent from the Leave side to build “a new government to bind the country together”.
Asked if victory for Leave might force an election in the autumn, Mr Rees-Mogg said: “I would not rule out a new election altogether … A new general election is not impossible.”
Work and Pensions Secretary Stephen Crabb said the Government must respond to “social divisions” which seemed to have pushed many of the “white working class” to vote Leave.
There were calls within Labour too for a change in direction, with Bassetlaw MP John Mann saying that the strong showing for Brexit in former heartland areas showed the party was “out of touch” with traditional voters who were “sick to death” with what they were being offered.
Shadow chancellor John McDonnell said it was clear “people don’t think we’ve been listening enough” and suggested Labour should reconsider its stance on freedom of movement.
Whatever the result, Mr Cameron would be a “hostage” to his pro-Brexit MPs who will make sure they seize “key positions”, said the shadow chancellor.
Senior Labour figures including Ed Miliband and Yvette Cooper suggested that the scale of support for Leave was fuelled by discontent with the way the country was heading on issues like wages, jobs and opportunities for the young as much as by opposition to the EU.
“It’s a nation divided and the PM will have a big responsibility – particularly if it’s a Remain win – to show he understands what people are saying on the Leave side of the argument,” said former Labour leader Mr Miliband. “Labour faces that responsibility too.”
Some 84 Leave-backing Conservatives signed the letter to the PM, as Tories battled to restore a unity riven by weeks of divisive “blue-on-blue” fighting.
In it they wrote: “We believe whatever the British people decide you have both a mandate and a duty to continue leading the nation implementing our policies.”
As well as Mr Johnson and Mr Gove, the signatories included Cabinet-level Brexit backers Chris Grayling, Theresa Villiers, John Whittingdale and Priti Patel.
But former Cabinet ministers Iain Duncan Smith, Owen Paterson, Cheryl Gillan and David Jones did not sign, along with the chair of the backbench 1922 committee Graham Brady and influential MPs including Mr Davis and Bernard Jenkin.
Labour’s Jonathan Ashworth said the Conservative Party was “utterly preoccupied with leadership infighting rather than the future of the country”, adding: “This letter cannot unsay what senior Tory politicians have been telling us for weeks – that the British people simply cannot trust David Cameron.”
Mr Grayling said: “It would be an absolute nonsense if David Cameron felt, having given the country that choice, if they take the decision he couldn’t carry on the job. We are completely behind him staying, we want him to stay and that letter is a statement of commitment to his leadership.”
But shadow foreign secretary Hilary Benn told the BBC: “If there were to be a vote to Leave, then as far as the Prime Minister is concerned I don’t see how he is going to remain in his job for very long at all.
“I think it’s very hard for him in those circumstances to remain. If you are the Prime Minister, you’ve called this referendum, you’ve laid your reputation on the line and your arguments, I think it’s going to be very hard.”
Lib Dem former Cabinet minister Sir Vince Cable said Mr Cameron’s authority would be “completely gone” in the event of a Leave win and he would have to stand down.
He described holding the referendum as a “very bad call” by the Prime Minister, who failed to understand what happens “when you just throw the cards in the air”.
A high turnout was expected in the referendum, despite torrential rain in South-East England which forced the closure of some polling stations and caused transport disruption for commuters planning to vote on their way home.
A record 46,499,537 voters were eligible to take part, said the Electoral Commission, meaning that a turnout of a little over 72% could surpass the highest number of ballots cast in a general election.
Press Association analysis of turnout figures suggested that either camp will need a total of around 16,800,000 votes to pass the winning post.
International Development Secretary Justine Greening said the result was still “too close to call”.
“I think Nigel Farage has really jumped the gun in what he has just said,” she told the BBC.
Whichever way the result went “we will need to knit our country back together and recognise that whoever wins, one in two people will have voted for the side that didn’t come out on top”, said Ms Greening.
