NICOLA Sturgeon and the SNP are set to sweep to a dramatic victory today with an exit poll pointing to the Nationalists taking 58 out of 59 Scottish seats as David Cameron is forecast to remain Prime Minister with the Conservatives on course to be the largest party on 316 seats.

The joint BBC/ITV/Sky snapshot, released as polling stations closed, pointed to Labour gaining just 239 seats, 17 fewer than their tally at the start of the election campaign while the Liberal Democrats were predicted to suffer a collapse from 56 in 2010 to just 10.

Such a result would likely lead to both Ed Miliband and Nick Clegg set to quit as the leaders of their respective parties.

However, the Tories' forecast of 316 seats was short of the 323 needed - when Sinn Fein, which does not take up its Commons seats, is taken into account - to secure a Commons majority.

This, if true, would throw up the possibility of another Con-Lib coalition or, possibly, a minority Conservative government, relying on either Lib Dem votes or Democratic Unionist votes in a confidence and supply deal.

Responding to the exit poll, Tory chief whip Michael Gove said: "It could be right, yes. If it is right it means the Tories have clearly won this election and Labour have clearly lost it."

The Scot fighting a Surrey seat said the predicted result would, if accurate, give Mr Cameron "considerable authority" and that this morning he would be due to make a Downing Street statement on how a Conservative government would "secure our economic recovery".

Tory Environment Secretary Liz Truss was more cautious, making clear the Conservatives should not get ahead of themselves based on the exit poll.

"It is certainly encouraging to see those results. It's far too early to speculate (on a government) because we don't know how that is going to translate precisely into a number of seats.

"What we are seeing across the country is different things happening in different parts of the country," she added.

A virtual clean sweep by the SNP in Scotland would leave all the opposition parties in disarray with possible Labour or the Liberal Democrats facing a wipe-out while it is also likely to mean that, in Westminster terms, Scotland will be once again a Tory-free zone.

However, SNP leader Nicola Sturgeon was also cautious about the exit poll, tweeting: "I'd treat the exit poll with huge caution. I'm hoping for a good night but 58 seats is unlikely!"

Labour deputy leader Harriet Harman also urged caution over the joint exit poll and declined to be drawn on the implications of such a result for Mr Miliband's position as leader.

"I have been on television where I have been commenting on exit polls in the past where the exit polls were wrong, because it is difficult to read these situations from exit polls," she told the BBC.

"We have had a really excellent campaign," she said, that was "very much in touch" with voters' concerns.

Asked if Mr Miliband could continue as leader if the poll proved correct, she said: "Well, at this stage we are just waiting for the counts and seeing whether or not David Cameron has got a House of Commons voted in which is prepared to give him a majority.

"You will forgive me for actually waiting until we have the answer to that big question before we go on to any other hypotheticals."

Meantime, Lord Ashdown, commenting on the exit poll, told BBC presenter Andrew Neil: "If this exit poll is right, Andrew, I will publicly eat my hat on your programme."

He insisted he had often seen "very inaccurate exit polls" during his long political career.

"You expect me to speculate on what I regard to be a certain error, I'm not going to do it," he said.

Questioned about whether the Lib Dems would form another coalition with the Tories, he said: "I am clear that the Liberal Democrats have an appetite for doing what they've done before, which is putting national interests first."

But a YouGov opinion poll of 6000 people who had voted painted a much brighter picture for Mr Miliband, putting Labour and Tories tied on 34 per cent each, Ukip on 12 per cent, Lib Dems on 10 per cent, the SNP and Plaid Cymru on five and the Greens on four.

This was much closer to polling during the campaign, with a final poll of polls compiled by the Press Association putting Tories on 276 seats, Labour on 271, Lib Dems on 28, SNP 48, Ukip three and Greens one.

If the exit survey turns out to be correct, it will spark an intense bout of soul-searching among the polling companies, who may have to reconsider their methods of gauging public opinion after their results suggested the election was the tightest in living memory.