David Cameron has warned of more borrowing and taxes if Ed Miliband does a deal with the Scottish National Party to become Prime Minister.

The Conservative leader said that Mr Miliband had failed to rule out a vote-by-vote deal with Nicola Sturgeon, following a TV debate in which the SNP leader repeatedly urged her Labour counterpart to join her in an anti-Tory alliance after the May 7 General Election.

Mr Miliband was judged the "winner" of the 90-minute showdown and viewers taking part in an instant poll favoured him as prime minister over Mr Cameron - who did not take part - by a margin of 45% to 40%.

The Labour leader concluded the BBC broadcast by issuing a direct on-air challenge to the PM to debate him head-to-head on TV before polling day.

The challenge was brushed off by the Conservatives who said the debate had shown that if Mr Miliband entered No 10 at the head of a minority Labour government it would be the SNP who would be "in the driving seat".

And Mr Cameron said in a Twitter message to voters: "Ed Miliband won't rule out a vote-by-vote deal with the SNP so he can be PM. It would mean more borrowing and more taxes and you would pay."

The party leaders were returning to the campaign trail with Mr Miliband promising to end the "scandal" of long-term internships and Mr Cameron warning of the threat to the Conservative "jobs miracle" if Labour gained power.

Nick Clegg was heading to Gordon in Scotland where ex-SNP leader Alex Salmond is bidding to take the seat from the Liberal Democrats, with an appeal to Labour and Tory supporters to vote tactically to keep out the nationalists.

The debate saw Ms Sturgeon alternately attacking and wooing the Labour leader, as she urged him to take part in a progressive alliance which would make his policies bolder. But Mr Miliband resisted repeated attempts to draw him into agreement on an arrangement to work together in the event of a hung parliament.

Ms Sturgeon warned Mr Miliband he would not be forgiven if he refused to work with the SNP in a hung parliament to "lock David Cameron out of Downing Street".

"We share a desire to see the back of the Tories but surely we do not want to replace the Tories with 'Tory-lite' - we want to replace the Tories with something better," she told him.

"Is it the case that you would rather see David Cameron go back in to Downing Street than work with the SNP? Surely that cannot be your position," she said.

Mr Miliband retorted there was a "huge difference" between Tory cuts and Labour's deficit reduction plans, adding: "I've fought the Tories all my life, unlike the SNP. You've fought Labour all your life, Nicola. I just don't buy it."

In a poll of 1,013 viewers conducted by Survation for the Daily Mirror, Mr Miliband came out on top in last night's debate, with 35% judging him the winner, narrowly ahead of Nicola Sturgeon on 31%.

Ukip leader Nigel Farage was third on 27% followed by the Greens' Natalie Bennett on 5% and Plaid Cymru leader Leanne Wood on 2%.

However Conservatives immediately claimed that the debate had exposed the extent to which a minority Labour government would be in thrall to the SNP.

Commons Leader William Hague said: "Nicola Sturgeon, it is very clear from this debate, wants to put Ed Miliband into Downing Street and then drive him into more and more extreme positions."

The Conservative leader came under attack for failing to attend the debate, with Mr Miliband saying he had "chosen not to defend his record".

Ms Sturgeon was applauded as she branded the Prime Minister's absence a "disgrace".

Liberal Democrat leader Nick Clegg, who complained that he had been excluded from last night's event, said he was ready to debate Mr Miliband "any time, any place, anywhere" even if Mr Cameron would not.

Ukip leader Nigel Farage, who repeatedly found himself isolated as other leaders condemned his calls to quit the EU and radically cut immigration, said he was "the only person here saying what a lot of people at home are really thinking".

And he described the audience in Westminster's Central Hall who applauded his opponents as "remarkable - even by the left-wing standards of the BBC" - something which prompted presenter David Dimbleby to insist that those attending had been independently chosen to represent the spread of opinion in the population.

Among Scottish viewers, Ms Sturgeon was judged to have performed best by a massive margin of 68%, to Mr Miliband's 17%.

The Prime Minister issued an appeal in the Brighton Argus newspaper for voters to support Conservatives to "stop Labour wrecking our country again".

Tories are defending marginal seats in Brighton Kemptown and Hove, and engaged in a three-way fight with the Greens and Labour in Brighton Pavilion, which gave the environmentalist party its first House of Commons seat in 2010.

Mr Cameron wrote: "A vote for anyone other than Conservatives could make Miliband prime minister. And the result would be clear: more taxes, more spending, more borrowing and more debt - all the things that got Britain into a mess in the first place."

He warned there was a "very real possibility" of Mr Miliband entering Downing Street "on the coat tails" of the SNP, adding: "It would be an alliance between the people who want to bankrupt Britain and the people who want to break up Britain - and it would be disastrous for every city, town and village in this country."

Shadow Scottish secretary Margaret Curran said she had never seen Ms Sturgeon make overtures to Labour before last night and so would be arguing for a majority.

The Labour frontbencher said: "I did an about take when I listened to that debate last because I've seen Nicola Sturgeon all of a sudden claiming that she's got some kind of joint interest with Labour.

"I've worked in Scottish politics for a long time and I've never heard Nicola Sturgeon make any positive remarks about Labour. She's argued against Labour all her political life.

"I can respect her argument because she believes fundamentally in independence, she has not ever shown any interest in Labour."

Mr Farage told the BBC News Channel: "It's completely clear that the only way that Ed Miliband can become prime minister is with the SNP's support.

"I am very worried about the Scottish tail wagging the English dog. I'm very worried as to what that means for English taxpayers. I'm very worried about what it means when it comes to immigration and all of these issues.

"I would just say, if you are a Labour voter who's worried about the kind of issues I'm worried about, be very, very careful before you vote."