The ComRes poll for The Independent on Sunday puts the Lib Dems on 29 per cent, two points ahead of Labour on 27 per cent and just two behind the Conservatives, who are on 31 per cent – their lowest rating since September 2007, when Gordon Brown was considering calling a snap election.
The survey, taken in the wake of the first TV debate last Thursday, shows how the election campaign has been dramatically changed and makes a hung parliament more likely.
Mr Clegg's party is up 13 points from last week's IoS poll, when they were languishing on 16 per cent. The Lib Dem figure today is an eight-point increase on the latest ComRes poll for The Independent.
Touring Kingston Hospital in Surrey yesterday with his wife Miriam, Mr Clegg said: "All bets are off."
The poll will cause alarm in David Cameron's team, with some insiders complaining that the Tory leader's decision to allow Mr Clegg to share a platform in the debate was a major tactical error.
After his faltering performance in the debate, Labour's third place is an embarrassment to Mr Brown.
But if replicated on polling day, the figures would still leave Labour with the largest number of seats, even though they have the third-highest vote share – due to the imbalance of the first-past-the-post electoral system. But no party would have an outright majority. According to the electoralcalculus.co.uk website, Labour would have 273 seats, the Tories would have 239 and the Lib Dems 106.
With a hung parliament on the cards, the poll shows 64 per cent of voters want Mr Clegg to play some part in the next government, while 68 per cent say he has as much credibility as the Conservative leader and the Prime Minister.
There is some good news for Mr Cameron, with 50 per cent saying they know what the Conservative leader stands for, up from 48 per cent last October.
But while the Conservatives retain their poll lead, the Cameron team has been unsettled by the Lib Dem surge, which has come at the expense of their party rather than Labour.
In an interview with The Independent on Sunday, the Conservative Party chairman, Eric Pickles, claims a hung parliament would be bad for Britain.
"I think the electorate will not want to see a hung parliament, I think they recognise that that will be expensive for the country, difficult for our economy and would make a very bad situation worse."
Asked if it would be a disaster for the Tories if they didn't win on 6 May, Mr Pickles said: "I would be awfully sad. It certainly would be a disaster for the country, to have that dithering Prime Minister making things worse, but as my old granny would have said, don't cross that bridge till you get there."
Writing in The IoS today, Labour's election chief, Peter Mandelson, exploits the Tories' unease at the decision to push for Britain's first ever leaders' debate.
Lord Mandelson says: "Under sustained questioning, Mr Cameron showed that he is fatally crippled in his attempt to set out a distinctive set of policies. That's because the Big Society has proved nothing more than a sticking plaster that is failing to cover up the ideological fault line in his own party.
"Unlike New Labour, the Tories have not hammered out a unifying philosophy that can be turned into a credible strategy for government."
On the campaign trail yesterday, Mr Cameron told activists in Gloucester the Tories had a "great fight on our hands". Significantly, he failed to mention Mr Clegg or the Liberal Democrats by name in an attempt to frame the election contest as a two-horse race between him and Mr Brown.
He said that only the Tories would "get the job done" of fixing Britain's economy and society.
"Is another five years of Gordon Brown going to get that job done? He has had 13 years and he is making things worse.
"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."
Mr Brown, speaking to supporters in Bedford, insisted he was the only leader of "substance".
He said: "As the dust settles after the first two weeks, people will have all their views about style and about presentation and about the images and personality.
"But, you know, an election like this is a big choice and it comes down to very, very big issues, and it comes down to the big choices people have to make about the big issues of the future."
Tories still ahead, but Clegg has raised doubts
Andrew Hawkins
ComRes, pollster for the IoS
"I now think the Conservatives will be short of a majority. The undecided vote and much of the minority parties' votes went flying over to Nick Clegg when people watched the debate on Thursday."
Predictions last week Con majority 32; now hung parliament (Con 11 seats short of a majority).
Ben Page
Ipsos-Mori
"The Tories will be the largest party but the seats are impossible to predict. The debates are a wild card. If the Lib Dems can maintain this support, things could be very different, but it's unlikely because they'll come under enormous scrutiny."
Predictions last week small Con majority; now Con lead, from 20 short of a majority to majority of 20 seats.
Andrew Cooper
Populus
"I'm standing by my view last week. Clearly, Nick Clegg won the first debate; but, more significantly, Cameron didn't lose it. My hunch is that the Lib Dems will get a long-term boost, but I think there's a strong chance they'll take more votes from Labour than the Tories."
Predictions last week Con majority of 10+ seats; now same.
Robert Salvoni
Harris Interactive
"We continue to predict a narrow Tory majority, as the swing from Labour in the marginal-rich regions is stronger than the national average. The dramatic 'TV debate effect' is likely to soften over time as other campaigning and media coverage hots up."
Predictions last week Con majority of 2-10 seats; now Con majority of 10.
Andy Morris
Angus Reid
"Nick Clegg was seen to be the clear winner in the debate. I expect his performance to boost the Lib Dems. This will make it harder for the Tories to take seats from the Lib Dems and may help them in key marginals. However, I still see Cameron in No 10."
Predictions last week Con majority of 40-50; now Con majority of 30-40.
Martin Boon
ICM Research
"I expect a boost to the Lib Dems. Nick Clegg did well being given an equal presence with the other leaders, which hasn't been there in previous elections. Whether that changes the number of Labour or Tory seats remains to be seen, but I'm revising down my projection."
Predictions last week Con majority of 20 seats; now Con majority of 15.
Johnny Heald
Opinion Research Business
"I still believe the Tories will win by the same amount. Nick Clegg did well in the debate, but the reality is he came from a position of a lot of people knowing very little about him. This is about who's going to make the best prime minister. He did really well, but I don't think it will be a hung parliament."
Predictions last week Con majority of 40+ seats; now same.
Graham Dumper
Marketing Means (pollster for the Liberal Democrats)
"Thursday's debate has become the 'water cooler moment', throwing the other two parties' strategies into disarray. The campaign was a big turn-off for many people before. The question remains whether the Lib Dems can drive this further."
Predictions last week hung parliament; Con 25 seats short of a majority; now same.
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