British Prime Minister Gordon Brown on Tuesday launched a month-long election campaign likely to be dominated by the economy and shaping up as the most unpredictable contest since 1992.

Standing outside his Downing Street residence in the sunshine, with his cabinet arrayed behind him, Brown ended weeks of shadow boxing by confirming the May 6 election date, just a month before the last possible time for an election.

“It’s probably the least well-kept secret of recent years but the queen has kindly agreed to the dissolution of parliament and a general election will take place on May 6,” Brown said, starting his bid to extend Labour’s 13-year grip on power.

The opposition Conservatives lead centre-left Labour in opinion polls. But the election result is far from certain as public support for both main parties remains volatile and the opposition faces a big challenge in securing power.

Polls point to a hung parliament in which no party has an overall majority because support for the centre-right Conservatives is less efficiently distributed across Britain’s 650 parliamentary constituencies.

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 firm action to tackle a budget deficit running at almost 12 percent of GDP.

Sterling fell more than one percent on the day versus the dollar, dragged down by political uncertainty.

“Sterling will now be at the whim of the opinion polls until the election on May 6. Polls showing no clear Conservative majority will be seen as negative for the pound,” said Michael Hewson, analyst, at CMC Markets.

OPENING PITCHES

Conservative leader David Cameron, addressing supporters on the banks of the River Thames opposite parliament, called it the most important election for a generation.

“You don’t have to put up with another five years of Gordon Brown,” said Cameron, 43, a former public relations executive.

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 straightened times.

The Conservatives promise to cut the deficit harder and faster than Labour but have also pledged to exempt most workers from a payroll tax rise Labour plans for next year.

The 59-year-old Brown, who served as finance minister for a decade before replacing Tony Blair mid-term in 2007, said the recovery was too fragile to be entrusted to the Conservatives.

“Britain is on the road to recovery and nothing we do should put that recovery at risk,” said Brown.

“Get the big decisions right, as we did in the last 18 months since the world recession, and jobs, prosperity and better standards of living will result,” he added.

With the exception of Labour’s landslide victory in 1997, no party has secured more than a five percent swing in the national vote at a general election since 1950. The Conservatives require a swing of 6.9 percent to secure an outright win.

The new parliament will meet on May 18, the government said. That is later than unusual to give new members of parliament more time to prepare for their career change. It would also allow longer for horse-trading if no party wins a majority.

The outgoing parliament has been tarnished by a scandal over members’ expenses. As many as 150 members of parliament are stepping down, many with reputations harmed by the scandal.

All the main parties have suffered and some analysts expect fringe parties and independent candidates to win extra votes.

The smaller opposition Liberal Democrats could also play a pivotal role if neither of the larger parties wins a majority.

Adding spice to the campaign will be Britain’s first live television debates — between Brown, Conservative leader Cameron and Liberal Democrat leader Nick Clegg.

(Additional reporting by Sumeet Desai and Caroline Copley; Editing by Jodie Ginsberg and Jon Hemming)