London: David Cameron is likely to remain British prime minister on Friday, after a long, testing campaign left the polls at a dead heat on the eve of Thursday's general election.

But he may only hold that title for a few weeks as caretaker PM, if Labour and the Scottish Nationalists can muster enough MPs between them to deliver a hostile parliament, or if his Liberal Democrat coalition allies desert him in what would be an extraordinary act of political backstabbing.

"This has been an incredibly tight election," political scientist Jack Blumenau said. "We are relatively certain that there will be a hung parliament – we put it at about 97 per cent likelihood.

"This is likely to be an unprecedented moment in British political history."

Results in the election are counted overnight after polls close at 10pm Thursday (7am Friday in eastern Australia), and the result is not likely to be known until well into the following day.

Labour is on track to record its lowest ever vote in Scotland, with the Scottish Nationalists poised as the kingmakers on the left, but the Liberal Democrats may survive an anticipated electoral mauling well enough to hand power back to Mr Cameron for a second term in coalition.

On the last day of the campaign, the parties reverted to their core messages.

Mr Cameron said his party would deliver "strong, stable government that continues with a long-term economic plan that is working", and warned that national unity would be put in peril by a Labour minority backed by the SNP.

"This is the election that will define this generation," he said. "Let's build on what we have done, don't go back to square one."

Labour leader Ed Miliband promised to "rescue our National Health Service, which needs rescuing", and to better look after working families' finances.

Liberal Democrat Nick Clegg continued to hammer his message of balance, claiming that his party could bring "heart to a Conservative government and brains to a Labour government" if it was given the strength to form a coalition with either of the major parties.

And SNP leader Nicola Sturgeon said her party was "within touching distance" of making history, so "the voice of Scotland is going to be heard more loudly at Westminster than it has ever been heard before".

She made it clear hers would be an anti-Cameron bloc, saying "we can make sure that the Tories are not simply replaced by a Labour Tory-lite government, they are replaced by something better".

According to one survey more than 4 million Britons plan to vote tactically – switching their vote from their preferred party in order to make sure the major parties hold off minor challengers such as the UK Independence Party (UKIP) and the SNP. Labour leaders have been wooing Lib Dem and Green voters, to keep out the Tories, while the Conservatives have urged UKIP and Lib Dem voters to vote Tory to stop the Labour surge.

There has also been a surge of interest in "vote swapping" sites on the internet, where people from different regions agree to exchange votes in order to have a bigger impact on the first-past-the-post system.

In promising signs for Labour and the minor parties, a final poll by Lord Ashcroft - a former deputy chairman of the Conservative Party - found that more than half of swing voters agreed that "the policies of the last few years have failed and it is time for a change".

Since February there had been a small but significant drop in the number of voters overall who thought the country was on the right track.

London School of Economics professor Bart Cammaerts said the Conservatives have fought a typical incumbent's campaign, "focusing on strong leadership and the economy, but they also waged a negative campaign, scaremongering and inducing fear of their opponents".

Professor Cammaerts said Labour's had been a more positive campaign, trying to shift the battle onto their favourite turf of inequality, cost of living and protecting the National Health Service.

"The majority of people in the UK do not consider that they are necessarily better off after five years of a Con-Dem coalition," Professor Cammaerts said. "After many years of austerity and hardship, many people are convinced the system does not work in their interest; that the few are profiting disproportionately to the detriment of the many."

In a survey at the beginning of the year, Professor Cammaerts found a changing political landscape, in which almost two-thirds of the British people believed inequality was at unsustainable levels, and almost three-quarters thought the rich and corporations should pay more tax.

However Labour failed to turn this into electoral advantage during the campaign, as it found itself constantly defending its record in office prior to 2010, when the country plunged into recession.

The election is not only significant for Britain. One of the Conservatives' key promises is to deliver an in-out referendum on the European Union, possibly within a year – which could reshape the country and the continent.

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