Britain's party leaders campaigned around the clock Tuesday in a final push for votes, two days before a parliamentary election that opinion polls suggest could redraw the political map.
Prime Minister Gordon Brown hinted that he could step aside if his Labor Party fails to win a fourth consecutive election Thursday, as most polls suggest.
Recent surveys have indicated that David Cameron, hoping to end his center-right Conservative Party's 13 years in opposition, would either win a slim majority in parliament or fall just short of it.
But a new poll showed the race tightening again.
The YouGov survey for the Sun newspaper showed Labor cutting the Conservatives' lead to five points while the Liberal Democrats, traditionally Britain's third party, slipped back. That outcome would make Labor the largest party in parliament, though without a majority, if repeated Thursday.
"I have never known so many undecided voters as we have seen in this election," Brown told a rally in Manchester.
Cameron planned to campaign overnight, with events scheduled in northern England early Wednesday as he seeks support from the third of voters said to still be wavering.
The rise of the Lib Dems has added to the unpredictability and turned the contest into a three-way fight.
The Lib Dems could hold the balance of power in an inconclusive election and will use that to push for a proportional voting system.
Brown, finance minister for a decade until 2007, indicated earlier he could step aside if Labor flops at the polls.
"I will take full responsibility if anything happens," Brown told GMTV, a breakfast television show. "But I still think there are thousands of people who have still to make up their minds."
Cameron accused Brown of lying about alleged Conservative plans to cut benefits, saying he had conducted the most negative campaign in modern British political history. "It's been the most disgraceful campaign," he told a rally in Scotland.
Brown's campaign was undermined by one of his candidates who described him in a local newspaper interview as "the worst prime minister ever."
Manish Sood, standing for election in Norfolk, eastern England, told Reuters he stood by his comments.
'He is the worst'
"He is the worst because he has made it (Labour) a business party and he has made a mess of the economy," 38-year-old Sood said. "We need to go back to the basics of how life was in the 1970s .... a time of true moral socialism."
The Financial Times newspaper gave Cameron a boost, switching its support from Labor, in power since 1997, to the Conservatives. It said Cameron's party will be better at tackling a record budget deficit and managing the recovery from the worst recession since World War Two.
Data released Tuesday showed factory activity grew at its fastest rate in over 15 years last month, but lending to consumers, home-buyers and businesses was weak, leaving question marks about the sustainability of recovery.
Two of Brown's senior ministers appeared to appeal to center-left Labor supporters in some close-fought electoral districts to consider voting for the Lib Dems to undermine the Conservatives.
Lib Dem leader Nick Clegg, who has shot to prominence in the campaign after strong performances in TV debates, repeated his ambition to be prime minister and said he was not just hoping for a close result that would heighten his party's influence.
"I am playing for victory," he told Sky News, saying that if millions more people voted for the Lib Dems, it would make the momentum toward the political and economic reforms they want increasingly unstoppable.