LONDON - Party leaders criss-crossed Britain in a frantic final day of campaigning Wednesday as they battled to win over undecided voters, before what is expected to be the closest election contest in decades.

Prime Minister Gordon Brown, David Cameron of the main opposition Conservatives and Nick Clegg of the Liberal Democrats all acknowledged that the election was still up for grabs.

The urgency of the last-minute campaigning was underlined by a poll showing that nearly four in 10 voters were yet to make up their mind who to back on Thursday.

In the face of polls showing the Conservatives ahead in the race, Brown said he was 'determined' and 'resolute' to keep Labour in government for a fourth term.

Brown told a crowd in Bradford, northern England, that a Conservative government would plunge the economy back into recession with their promises of deep cuts.

'Tomorrow is the day for us to stand firm for what everybody knows to be true ... I am determined and I am resolute and I am fighting not for me but for Britain's future,' he said.

Conservative leader David Cameron campaigned through the night, talking to fishermen, ambulance drivers and shift workers in marginal seats in northern England that the Tories must win to take back power.

Aides said the 24-hour blitz was Cameron's answer to his opponents' claim that he is complacent and was taking victory for granted.

'I never believed this election was going to be easy,' Cameron told GMTV early Wednesday.

'The British people don't hand you the government of the country on a plate. Quite rightly, they are making us work for it,' he said.

Clegg was making a last dash for votes across England in Eastbourne, Durham and Sheffield.

He told voters that by choosing his party they had a chance to avoid a 'stitch-up' by the two main parties.

The latest polls showed the Conservatives would win the most seats, but not enough to form a government on their own, leading to a hung parliament.

Three surveys showed the Conservatives ahead. A ComRes poll for ITV News and The Independent newspaper showed the Tories ahead on 37 percent, Labour on 29 and the Liberal Democrats 26.

But the same poll also indicated that nearly 40 percent of voters were still undecided.

Clegg said: 'There is a still a day to go, there are still lots of people who haven't decided how to vote.'

Brown refused to endorse calls from some Labour cabinet ministers that their supporters should vote for the Liberal Democrats in seats where Labour had no chance, to stop the Conservatives from taking power.

'I want every Labour voter to vote Labour. I want that because I want a majority Labour government,' Brown told BBC radio.

In an interview with The Times, he refused to be drawn on whether he would consider forming a coalition government with the Liberal Democrats, saying: 'I'm not going to talk about what happens after Thursday.

'The people are the boss. The people make decisions. No politician should be speculating about what the people are going to say.'

Clegg, whose party's surprisingly strong performance has made the election so close, dismissed talk of tactical voting as 'another sign of Labour's desperation.'

His party has enjoyed a surge of support on the back of his performance in TV debates, and he is anxious to avoid tactical voting by his own supporters, fearing it could dent his party's share of the popular vote.

This would damage his negotiating power in the event of a hung parliament, in which he could hold the balance of power.

In the latest celebrity endorsement, television and music mogul Simon Cowell backed Cameron, writing in The Sun: 'I believe he is the prime minister Britain needs at this time.

'He has substance and the stomach to navigate us through difficult times.'

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