By Guy Faulconbridge and Andrew Osborn

LONDON (Reuters) - British voters get to decide on Thursday who they want to rule the world's fifth-largest economy in a tight election that could yield weak government, propel the United Kingdom towards a vote on EU membership and stoke Scottish desire for secession.

Prime Minister David Cameron's Conservatives and Ed Miliband's opposition Labour Party have been neck and neck in opinion polls for months, indicating neither will win enough seats for an outright majority in the 650-seat parliament.

"This race is going to be the closest we have ever seen," Miliband told supporters in Pendle, in northern England, on the eve of the vote. "It is going to go down to the wire."

Cameron said only his Conservatives could deliver strong, stable government: "All other options will end in chaos."

The Conservatives portray themselves as the party of jobs and economic recovery, promising to reduce income tax for 30 million people while forcing through further spending cuts to eliminate a budget deficit still running at 5 percent of gross domestic product.

Labour says it would cut the deficit each year, raise income tax for the highest 1 percent of earners and defend the interests of hard-pressed working families and the treasured but financially stretched national health service.

If neither wins an overall majority, talks will begin on Friday with smaller parties in a race to strike deals.

That could lead to a formal coalition, like the one Cameron has led for the past five years with the centrist Liberal Democrats. Or it could produce a fragile minority government making trade-offs to guarantee support on key votes.

British stock and bond prices were moderately higher on the final trading day before polling, with election jitters still failing to register any significant investor selling.

The pound was slightly weaker but remained within the relatively tight trading ranges seen over the past three months.

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"What is important to us is clarity as soon as possible," Andrew Witty, chief executive of GlaxoSmithKline, Britain’s sixth largest company by market value, told reporters.

'DISUNITED KINGDOM'

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.

Leading pollster Peter Kellner of YouGov predicted the Conservatives would end up with 284 seats to Labour's 263, with the Scottish Nationalists on 48, Liberal Democrats 31, the anti-European Union UK Independence Party (UKIP) two, Greens one, and Welsh and Northern Irish parties 21.

If that proved correct, either of the two big parties would need support from at least two smaller ones in order to get laws through parliament.

If a durable government could not be formed, Britain could face political instability and even possibly a second election.

At heart, Britain's post-World War Two political consensus, which saw the Conservatives and Labour take turns in government, is crumbling as once marginal parties in Scotland and England steal millions of votes from established parties.

Scottish nationalists, who lost an independence vote last September, are likely to win the lion's share of seats in Scotland, capturing dozens from Labour, and making Miliband's chances of winning an overall majority much slimmer.

In England, UKIP has courted Conservative and Labour voters, but is likely to do most damage to Cameron's chances of a majority.

Cameron has warned that Miliband will only be able to rule with the help of the Scottish National Party (SNP), a result Cameron has warned could place the United Kingdom in peril.

Miliband has ruled out doing a deal with the nationalists, despite their overtures to form an alliance against the Conservatives.

"If we work together we can lock out the Tories. We will work with others across the United Kingdom, that is my pledge," SNP leader Nicola Sturgeon said in Edinburgh.

If Cameron fails to win a majority, he could try to strike a deal with the Liberal Democrats, a repeat of the 2010 coalition, and possibly also with Northern Irish unionists and UKIP.

Cameron has promised an in-out referendum on Britain's membership of the European Union by the end of 2017.

Polls open at 0600 GMT for the United Kingdom's 48 million voters and close at 2100 GMT. An exit poll will be published as soon as polls close, and most results are expected in the early hours of Friday.

(Editing by Mark Trevelyan and Christian Plumb)