Gove: Labour government propped up by the SNP would be a lethal cocktail

"We saw in Nicola Sturgeon, certainly an impressive performance but also a performance from someone well to the left of not just the centre ground of British politics, but well to the left of Ed Miliband," Mr Gove told Sky News.

"I think it is a leap that is unmerited by the polls to say that a SNP-Labour government would be a popular choice in this country.

"I think that people appreciate that there would be an inherent instability in that arrangement.

"I don't think that people would like the potential chaos that would ensue if you had Ed Miliband as prime minister having to make every decision with Nicola Sturgeon and Alex Salmond auditing it to decide whether or not that decision was in the interests of Scotland and Scottish nationalism rather than the whole of the United Kingdom.

"If the country chooses to it could vote for a patchwork coalition ... I prefer to say a lethal cocktail of different parties which all have different objectives - there would be an automatic instability."

His comments came after a clutch of instant polls suggested that the one encounter of the General Election between Mr Miliband and David Cameron had proved inconclusive - although Ms Sturgeon was widely seen to have done well.

The Scottish First Minister deliberately sought to reach out to voters in England, saying that SNP MPs at Westminster would be "a voice to help bring about change for you too."

For Labour, shadow energy secretary Caroline Flint accused the Conservatives of deliberately talking up the SNP because of the threat they posed to Mr Miliband's chances in Scotland.

"They would love Nicola Sturgeon to do well because that puts David Cameron back in No 10," she told the BBC Radio 4 Today programme.

"At the end of this campaign it's about a choice who's going to be the prime minister and that's between Ed Miliband and David Cameron.

"If Scots don't want David Cameron, then they should think very carefully about voting for the SNP.

"We have made it very clear that we are not going to have a coalition with the SNP because we are fighting to win this election."

Liberal Democrat campaign chief Lord Ashdown said the debate showed that - in an election race which looked unlikely to produce an overall winner - only the Lib Dems could provide stable coalition partners in a hung parliament.

"You have got three choices. You have got the SNP working with Labour determined to break up the country, you have got the mad, right-wing, far fringes of extremism of Mr Farage or you have got a Nick Clegg," he told the Today programme.

"If you accept the case there is going to be a coalition and no-one is going to govern by themselves, the choice between these three seems to me a no-brainer."

Ukip immigration spokesman Steven Woolf defended comments by his leader, Nigel Farage, highlighting the high proportion of HIV cases diagnosed in the UK who were not British nationals.

Plaid Cymru leader Leanne Wood drew cheers from the studio audience when she accused the Ukip leader of "scaremongering" and told him that he should be ashamed of himself.

Mr Woolf told the Today programme: "Nigel is not saying that those people should not be treated now.

"What he is saying is that the National Health Service should not be an international health service.

"It is a question of whether we should be having an insurance policy for those people who are travellers, visitors, coming here as students, as happens in over a hundred countries across the world, including places like the United States or Canada.

"No-one criticises Obama for having a similar sort of provision in place, it is only because Nigel is saying it and because he is being so successful at the moment."

On the night, Mr Farage claimed an early advantage in the instant polls, with 24% of viewers polled at the halfway point by ComRes for ITV News rating him the best performer, ahead of Mr Miliband on 21% and Mr Cameron on 19%.

But by the end, the picture was less clear, with Mr Miliband leading an ICM poll in The Guardian taking 25% of support, just ahead of Mr Cameron on 24%, with Mr Farage on 19%.

A ComRes post-debate poll for ITV News had Mr Miliband, Mr Cameron and Mr Farage tied in first place on 21%, with Ms Sturgeon on 20%.

A YouGov poll had the SNP leader top with 28% backing, followed by Mr Farage (20%), Mr Cameron (18%) and Mr Miliband (15%).

Mr Clegg, who was launching a poster highlighting the tax cuts delivered by the Lib Dems in Hyde, Cheshire, said the debate showed that politics was becoming more fragmented.

He said it was "for other people to judge" whether he got his message across but added: "The fact that there were seven there just shows that politics is becoming more fragmented.

"The only people who don't realise that are Ed Miliband and David Cameron, they think it's still a game of pass the parcel between the two old parties.

"I actually think that most people watching last night will have realised that no-one is going to win this election outright, so it's all about who is going to work with who.

"And of course the Liberal Democrats have shown in the last five years that we can work in coalition to anchor the government in the centre ground to make sure that we act fairly for the whole of the country and that we don't lurch this way or that."

Mr Clegg added: "I certainly hope on my part that I was able to get the message across for the Liberal Democrats."

The Lib Dem leader explained why he took on Mr Cameron over the Conservatives' plans for further cuts: "I think they need to be called out because they are going around the country saying stick with the plan and they have absolutely no intention of sticking to the plan themselves at all."

Tories scooped their best poll rating in a year in a YouGov survey for The Sun which put Mr Cameron's party on 37%, with Labour also coming close to their recent peak with a rating of 35%.

The poll of 1,583 adults taken on April 1 and 2 appeared to confirm a slight squeeze on minor parties as the campaign has progressed, with Ukip on 12%, Liberal Democrats 7% and Greens 5%.

Campaigning in Edinburgh, Ms Sturgeon denied that a strong showing for the SNP in the election would let the Conservatives back into government.

"A matter of simple arithmetic says if the SNP and Labour combined have more seats than the Tories we lock the Tories out of government," she told Sky News.

"If the SNP's a big force in Westminster we can make sure that a Labour government doesn't sell out on its values in the way that the last Labour government did.

"We can make sure that we keep them honest and make sure they deliver real change for people."