David Cameron warned yesterday that a hung parliament would cripple hopes for change amid more signs of a surge to the Lib Dems.

With polls suggesting the historic first prime ministerial debate could have blown the general election wide open, Cameron insisted only a "decisive" Tory government could "get the job done".

The comments came after daily YouGov research for the Sun found the Lib Dems had leapfrogged Labour into second place after Nick Clegg's much-praised performance.

The party was on 30% to Labour's 28%, with the Conservatives in the lead on 33%.

It was the second poll to show a "bounce" in the wake of the debate, following an ITV/ComRes survey on Friday which put the Lib Dems up three points to 24%, but still trailing Labour on 28% and the Tories on 35%.

Addressing activists in Gloucester yesterday morning, Cameron said Britain was in desperate need of action to remove the "black cloud" of the deficit and clean up politics.

"Is another five years of Gordon Brown going to get that job done? He's had 13 years and he is making things worse," Cameron said. "Is a hung parliament going to get that job done? A hung parliament would be a bunch of politicians haggling, not deciding. They would be fighting for their own interests, not fighting for your interests. They would not be making long-term decisions for the country's future, they would be making short-term decisions for their own future. The way we are going to get things done is to have a decisive Conservative government."

Cameron did not refer to Nick Clegg or the Lib Dems explicitly, an indication that the Tories are taking the impact of the debate very seriously.

Not since the days of the SDP/Liberal Alliance in the 1980s has Labour been forced into third place in national polls of this kind.

Polling evidence suggests the swing to the Lib Dems was even stronger among the 10 million voters who tuned in to watch Thursday's debate on ITV.

The ITV/ComRes survey found that, of a sample of 4,000 viewers who watched the broadcast, some 35% said they would vote Lib Dem, against 36% for the Tories and 24% for Labour.

And Clegg was picked as the winner of the debate in four separate instant surveys, racking up support as high as 61% in one poll.

Thanks to the complexities of the voting system, the figures in the Sun poll may be more disastrous for the Tories than for Labour.

According to the BBC's election calculator, if repeated on an even swing across the country, they would produce a hung parliament with Labour the largest party with 276 MPs against 246 Tories and 99 Lib Dems.

Such a result would be certain to increase the pressure for electoral reform.