David Cameron has warned of more borrowing and taxes if Ed Miliband does a deal with the Scottish National Party (SNP) 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 45pc to 40pc.

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.'