Clegg is viewers' favourite in second leaders' debate

But instant polls of viewers of the 90-minute Sky News debate suggested they had not done enough to derail the Lib Dem leader's drive for Downing Street which gathered pace after a clear victory in last week's debate.

Mr Cameron came out on top in a YouGov survey for The Sun, with 36% of the 1,110 people questioned saying he won the debate, against 32% for Mr Clegg and 29% for Mr Brown. But a ComRes poll of 2,691 viewers for ITV News made Mr Clegg the winner on 33%, with the other two leaders neck-and-neck on 30%.

And Mr Clegg's party came out top in the ComRes poll when viewers were asked which way they will vote on May 6, with 36% choosing the Lib Dems, against 35% for Tories and 24% Labour.

If repeated on an even swing this would produce a hung parliament with the Tories as the largest party with 277 seats, Labour with 186 and the Lib Dems 160.

Both the Labour and Tory leaders took a more combative approach to the debate in Bristol, after being criticised last week for giving Mr Clegg an easy ride.

Rather than repeating "I agree with Nick" in a bid to win the Lib Dem leader over to their side, at one point Mr Cameron even said "I agree with Gordon" over nuclear weapons.

Mr Cameron launched a furious broadside at the Prime Minister over Labour leaflets which he said were spreading "lies" that a Tory administration would cut benefits for the elderly, forcing Mr Brown to claim that he had not "authorised" the mailshots.

But after his victory in last week's debate gave his party a boost of up to 12 points in the polls and turned the election into a genuine three-horse race, it was Mr Clegg who came under most pressure over his policies on Europe, Trident and immigration.

As the Lib Dem leader repeated last week's attack on MPs from "the old parties" who "flipped" their second homes to maximise their income from expenses, Mr Cameron stepped in to warn him not to place himself "on a pedestal".

Both Mr Brown and Mr Cameron accused the Lib Dems of planning an "amnesty" for illegal immigrants, while Mr Clegg said that the other parties were "in denial" over the impossibility of deporting hundreds of thousands of migrants who have settled in the UK for a decade or more.

And Mr Clegg came under attack for his proposal to include Britain's Trident nuclear deterrent in the strategic defence review which all three parties have promised to hold after the election.

The Lib Dem leader cited a group of retired generals who have warned that going ahead with replacing Trident could take money away from troops on the frontline, and said that US President Barack Obama had identified the threats of the future as terrorism and failed states, against which nuclear arms could not be used.

But Mr Brown told him: "I have to deal with these issues every day and I say to you, Nick, get real."

Mr Clegg had to "get real about the danger" of a nuclear-armed Iran and North Korea, said the Prime Minister.

Mr Cameron said: "I have never uttered these words before, but I agree with Gordon: You can't put off this decision."

The Tory leader said: "I profoundly believe that we are safer having an independent nuclear deterrent in an unsafe world ... a proper replacement for Trident."

Mr Brown accused the other two leaders of "squabbling" like children as they clashed over Europe. Mr Cameron dismissed the Lib Dem call for a vote on whether Britain should quit the EU as "a con", while Mr Clegg accused the Tories of allying themselves with "nutters, anti-Semites, people who deny climate change exists, homophobes" in the European Parliament.

"These two guys remind me of my two young boys squabbling at bathtime, squabbling about referendums on the EU when want we need is jobs and growth and recovery," said Mr Brown.

"I'm afraid David is anti-European, Nick is anti-American and both are out of touch with reality."