Speaking at the Conservative spring forum, former party leader William Hague said Britain could not afford to extend Labour's 13-year hold on power.
"It will be too late in five years' time to say we should have got rid of them, too late to reverse the decline: the debt will be too big, the bureaucracy too bloated," Hague told party activists in the southern English city of Brighton.
The Conservatives had looked assured of winning an election due by June, but their poll ratings have slipped. That has prompted talk of a hung parliament in which neither major party has an outright majority - a scenario that worries markets seeking swift action to cut a record budget deficit.
The Conservatives have said they would reduce the deficit more rapidly than Labour but Conservative leader David Cameron has recently played down talk of immediate "swingeing cuts.”
Prime Minister Gordon Brown, addressing the Welsh Labour conference, mocked the Conservatives as "the party that keeps changing their minds.”
Figures published on Friday showed the economy grew by 0.3 percent in the last three months of last year, ending a deep and painful recession.
Brown says cuts to a budget deficit forecast to top 12 percent of GDP must be delayed until recovery is assured.
"We will not put at risk the recovery and the jobs, the businesses and the homes of thousands of people who would be the direct victims of immediate Conservative cuts coming at the worst possible time," Brown said in a speech in the Welsh city of Swansea.
In an effort to boost business, the Conservatives would cut corporation tax to 25 percent from 28 percent with effect from April 2011 at the latest if they win the election, a Conservative source said on Saturday.
The Conservatives denied a report in the Guardian newspaper that leader David Cameron had set up a special unit to prepare for the possibility of a hung parliament in an election expected to be held on May 6.
Hague, the party's foreign affairs spokesman, said Brown's record as a prime minister was "the worst of modern times.”
He urged voters not to support the smaller opposition Liberal Democrats or smaller fringe parties that could benefit from an expenses scandal that tarnished the three main parties.
"The only vote for change in this election is a vote for a Conservative candidate," Hague said.
Outside in the seaside city of Brighton, people out shopping or enjoying some welcome winter sunshine had few kind words for politicians.
An amusement arcade proprietor said he had seen precious few signs of economic recovery.
"We've had some really bad weather. Everyone is feeling a bit run down," he said, casting his eye along the quiet seafront.
"I haven't made my mind up how to vote. I am a bit concerned about how the cuts in health care and education could affect my family."
