Britain's perennial third party, the Liberal Democrats, promised to tackle the country's crippling deficit and overhaul foreign policy if they achieve the near impossible and win next month's national election.
The party, which has 63 House of Commons seats, announced its platform on Wednesday with a vow to make sharp cuts to spending, break up major banks and build a less "subservient" relationship with the United States.
Nick Clegg, the party's 43-year-old leader, says he hopes to withdraw troops from Afghanistan by 2015, would oppose military action on Iran's disputed nuclear program and authorise a judge-led inquiry into allegations Britain colluded in the torture overseas of suspected extremists.
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His party usually commands about 20 per cent of British votes, and currently trails both Prime Minister Gordon Brown's Labour Party and the main opposition Conservative Party in opinion polls.
Clegg's economic spokesman Vince Cable said Britain's STG167 billion ($A276.6 billion) deficit is "the elephant in the room" ahead of the May 6 election, pledging the party would quickly reduce government borrowing.
"The two old parties, they are conspiring to airbrush the recession out of this election," Clegg said, launching the party's platform in London's financial district.
Britain has emerged from an 18-month recession - in which about 1.3 million people were laid off - but the nation's debt burden has risen dramatically after government programs to bail out imperiled banks and stimulate the economy.
Latest polls ahead of the election show Brown's Labour Party closing in on David Cameron's Conservatives.
A Populus poll for the Times newspaper put the Conservatives on 36 per cent - a drop of 3 percentage points - to Labour's 33 per cent. The Liberal Democrats had 21 per cent. The margin of error was plus or minus 2.5 percentage points.
A ComRes poll for the Independent newspaper and ITV television put Cameron's party at 36 per cent, down 1 percentage point, Labour up 1 percentage point to 31 and Clegg's party on 19 per cent.
The margin of error was plus or minus 3 percentage points.
Both polls indicated Brown's party would hold the largest number of seats in a hung Parliament, a break with recent polls that show Cameron is likely to either win a small majority, or have the largest hung Parliament grouping.
Other surveys show a lead for Cameron of 10 percentage points.
Clegg's party will likely play a pivotal role if both major parties are denied an outright majority - but he declined to say if the Liberal Democrats would offer support to one of their larger rivals in a coalition or voting pact.
"By definition, it would be new territory and politicians would need to talk to each other," Clegg said.
