LONDON (Reuters) - British Prime Minister Gordon Brown was set to announce on Tuesday a May 6 parliamentary election which could bring down the curtain on 13 years of rule by his center-left Labor Party.

Britain's Prime Minister Gordon Brown is driven away after giving evidence to the Iraq Inquiry at the Queen Elizabeth II conference centre in London March 5, 2010. REUTERS/Andrew Winning

Brown will meet Queen Elizabeth on Tuesday to request a dissolution of parliament, a Labor party source said, a formality which will mark the start of a month-long campaign for one of the most unpredictable elections in Britain for almost two decades.

The opposition Conservatives lead Labor in opinion polls but the gap has been narrowing. An ICM poll in Tuesday’s Guardian newspaper showed Labor only four points behind the Conservatives and on course to remain the largest party, albeit without an overall majority. [nUKPOLLS10].

Support for the Conservatives is unevenly distributed in Britain’s 650 parliamentary constituencies, meaning Labor can win the most seats even if it does not capture the largest share of the vote nationally.

An inconclusive election result is rare in Britain and is the nightmare scenario for financial markets, which want a clear outcome and the promise of meaningful action to tackle a budget deficit running at almost 12 percent of GDP.

Failure by either of the main parties to win a majority could hand a pivotal role to the smaller opposition Liberal Democrats, who will be trying to maintain a bloc of around 60 MPs in parliament.

ECONOMY TAKES Center STAGE

How best to run an economy slowly emerging from the worst recession since World War Two is likely to be the central theme in the campaign, entwined with issues such as how best to manage public services in straitened times.

Labor argues that Brown has steered Britain through turbulent economic times and to hand over now to an inexperienced opposition would jeopardize recovery.

“The people of this country have fought too hard to get Britain on the road to recovery to allow anybody to take us back on the road to recession,” Brown said in a statement previewing his campaign themes.

The Conservatives, led by former public relations executive David Cameron, have long said they would cut the deficit harder and faster than Labor but have now promised to exempt most workers from a rise in payroll tax that Labor plans from 2011.

That has won applause from the party’s traditional business supporters but Labor has cried foul, saying the Conservatives’ plans do not add up.

“We’re fighting this election for the Great Ignored. Young, old, rich, poor, black, white, gay, straight,” Cameron said in a statement, showing how he has tried to make his party more inclusive.

The outgoing parliament, which has served a full five years, has been tarnished by a scandal over lawmakers’ expenses that angered Britons. As many as 150 members of parliament are stepping down, many with reputations harmed by the scandal.

All three main parties have suffered and some analysts expect fringe parties and independent candidates to pick up extra votes at the polls.

Adding spice to the campaign will be an innovation in British politics -- live television debates between Prime Minister Brown, Conservative leader Cameron and Liberal Democrat Nick Clegg.