Nigel Farage has hailed a victory for Leave in the historic EU referendum, declaring "let June 23 go down in our history as our independence day".

The Ukip leader's judgment was endorsed by the major broadcasters, as ITV, the BBC and Sky News all calculated that, with less than 70 voting areas left to declare, Leave had built up an unassailable lead over Remain.

READ MORE: EU referendum results: Voting showing divide between Scotland and England

Sterling suffered one of its most precipitous plunges in the overnight markets, hitting lows last seen in 1985 and losing more than 10% against the US dollar, as traders responded with panic to the prospect of the UK quitting the European Union after 43 years.

The shock defeat for David Cameron's strong recommendation of a Remain vote led to questions about his future as Prime Minister.

Labour said he should "seriously consider his position" if Leave won, while Mr Farage said he should quit "immediately".

READ MORE: EU referendum results: Voting showing divide between Scotland and England

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 the PM's future. 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". And Conservative backbencher Jacob Rees-Mogg said a general election in the autumn was "not impossible".

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.

Shadow chancellor John McDonnell said the Bank of England may have to intervene to shore up the currency. Mr Cameron would now be "a hostage" to his pro-Brexit MPs, who will make sure they seize "key positions", he said.

Senior Labour backbencher and former Europe minister Keith Vaz said the EU should call an emergency summit to deal with the aftermath of the vote, which he described as "a crushing, crushing decision ... a terrible day for Britain and a terrible day for Europe".

Strong performances for Leave in the English shires, Wales and north eastern towns and cities 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. With a few dozen of the 382 voting areas left to declare, Leave had built up a lead of more than 1 million votes and looked set to win by a margin of 52%-48%.

The result in Scotland is certain to create massive pressure for a second independence referendum. Former Scottish National Party leader Alex Salmond said he was "quite certain" that an effort to drag Scotland out of the EU against its will would lead to the invocation of a manifesto promise to stage another ballot if there was a "significant and material" change in circumstances from the 2014 vote.

First Minister Nicola Sturgeon said Scotland had delivered a "strong, unequivocal vote" making clear that "the people of Scotland see their future as part of the European Union".

The Remain side was undermined by poor results in many of Labour's traditional strongholds in the north of England, including Sheffield, where Mr Farage hailed a narrow victory for Leave as "amazing". In a surprise result, Birmingham voted to Leave by the slimmest of margins.

The results sparked calls from within Labour 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.

Mr McDonnell acknowledged it was clear "people don't think we've been listening enough" and suggested Labour should reconsider its stance on freedom of movement.

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.

Former Labour leader Mr Miliband said the nation was divided and the PM had a big responsibility to show he understands what people are saying.

But he added: "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".

Some 72.2% of the 46,499,537 eligible voters turned out at polling stations, said the Electoral Commission.

READ MORE: EU referendum results: Voting showing divide between Scotland and England