By Andrew Osborn and Mark Trevelyan
LONDON (Reuters) - A late burst of opinion polls on Wednesday suggested that Conservative Prime Minister David Cameron and Labour challenger Ed Miliband had fought each other to a standstill on the eve of Britain's most unpredictable election in a generation.
Despite five weeks of campaigning, neither party has opened up a clear lead in the polls, pointing to a potentially messy and uncertain outcome from Thursday's vote.
The stakes are high because Britain's future in the European Union, as well as its national cohesion, could hinge on the result.
Cameron has promised to hold a referendum on whether to stay in the EU if he returns to power, while polls suggest Scottish nationalists could emerge as the third-largest party, despite losing a plebiscite last year on whether Scotland should break away from the United Kingdom.
Both the main party leaders have insisted they are fighting to win outright, despite the overwhelming signals from the opinion polls that they would need to seek one or more allies to form a coalition or survive as a precarious minority government.
"We can achieve an overall majority that gives Britain the strong stable government that continues, with a long-term economic plan that is working," Cameron said on Wednesday.
Miliband said his message to undecided voters was: "You can have another five years of a prime minister who will put the rich and powerful first in our country. Or if I'm prime minister, I'll put working people first."
Of seven opinion polls released on the last day before voting, three showed the two main parties tied. Three put the Conservatives ahead by a single percentage point, and one gave Labour a two-point lead.
Five years ago, Britain got its first coalition government since World War Two when Cameron fell short of an overall majority and struck a deal with the centrist Liberal Democrats to govern together to steady the economy.
Many Britons thought that was a one-off.
But the rise of fringe parties such as the pro-independence Scottish National Party and the anti-EU United Kingdom Independence Party (UKIP) has drained support from the two main parties.
The two men vying to become prime minister have avoided game-changing gaffes but drawn mockery at times: Cameron for forgetting the name of his soccer team, and Miliband for engraving his campaign pledges on a giant stone, prompting ironic comparisons with Moses.
UKIP, which has spent much of the campaign fending off accusations of racism, was forced to suspend one of its candidates on Tuesday for threatening to "put a bullet" in his Conservative rival, a Briton of Asian origin, if he ever became prime minister.
Stagnant polls have prompted Cameron to refine his message, blending the promise of higher living standards with a warning that Scottish nationalists could hold to ransom a minority Labour government, pushing it to borrow more and edge it towards another referendum on Scottish independence.
Awkwardly for the prime minister, a leading think tank cut its forecast for Britain's economic growth in 2015 on Wednesday, though it said strong consumer spending should keep the recovery on track.
But other data showed Britain's huge services sector unexpectedly picked up speed in April, countering signs of a slowdown.
The tightness of the race has prompted some parties to try to define in advance what arrangements would constitute a legitimate government.
The rules state that any party capable of getting parliament's backing for its legislative slate can govern. But convention dictates that the party with the most seats has a first attempt at trying to form an administration.
If the two main parties are separated by only a handful of seats, however, both may claim the right to govern, setting up a potentially protracted standoff.
Britain's highly politicised press weighed in with the Labour-supporting Daily Mirror warning its readers that Cameron would axe nurses at two thirds of hospitals, and the Conservative-supporting Sun featuring an unflattering picture of Miliband eating a bacon sandwich.
"Don't swallow his porkies and keep him out," it said.
More than 45 million Britons are eligible to vote on Thursday, when polls open from 0600 to 2100 GMT.
(Additional reporting by Kylie MacLellan, William James, Andy Bruce, Estelle Shirbon and Kate Holton; Editing by Susan Fenton)