Ed Miliband could be propelled into Downing Street even if he loses the election – by the Scottish nationalists.

Their leader, Nicola Sturgeon, yesterday threatened to defy the verdict of voters in most of the UK, doing a deal with Labour even if it has fewer seats than the Conservatives.

She also suggested she would demand a debt-fuelled £180billion public spending spree if she holds the balance of power at Westminster.

The SNP surge, which looks set to see it make an unprecedented Commons breakthrough in May, is fast becoming the major factor of the general election.

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Powerful partner: With the SNP willing to do a deal with the Labour Party, its leader Ed Miliband (pictured) could become the country's next Prime Minister even if the party lose the 2015 general election

Panic is mounting within the Labour Party at a recent poll by Lord Ashcroft suggesting it is facing a meltdown in Scotland at the hands of the SNP – to which it could lose 35 seats.

Calls are growing for Labour's 40 Scottish MPs, including its election chief Douglas Alexander, to head North to try to ensure they are re-elected.

Miss Sturgeon, Scotland's First Minister, yesterday said SNP MPs will do everything in their power to kick David Cameron out of office if there is another hung parliament.

No party which came second in a general election has formed a government since 1924, but the constitution does not prevent an alliance of smaller parties joining forces to form a majority.

Yesterday, making her third visit to London in as many weeks, Miss Sturgeon set out further conditions for propping up a minority Labour administration.

She claimed that austerity had failed and called for a £180billion spending boost across the UK – derailing plans to tackle the deficit.

She even demanded a return to 'Old Labour' socialist-style policies, urging a 'more progressive' approach that would reverse attempts by the Coalition to get the benefits bill under control.

Last night, the Tories accused the Scottish nationalists of attempting to 'bankrupt' the UK.

Motive: Scotland's First Minister and SNP leader Nicola Sturgeon (right) has said her party will do everything in their power to kick current PM David Cameron (left) out of office

Conservative Party chairman Grant Shapps said: 'Now we know the price that the SNP would charge to support Ed Miliband ... a bill of £180billion paid for by hard-working taxpayers.'

The Tories' chief whip in Scotland, John Lamont, added: 'First the SNP tried to break up Britain, now it wants to bankrupt Britain.'

Miss Sturgeon, who insists she would lead post-election negotiations rather than former leader Alex Salmond, has ruled out any deal with the Tories.

In an interview with Sky News, she was asked specifically if she could 'support Ed Miliband in Downing Street even if he led the second largest party?'

Miss Sturgeon replied: 'If he was able with the support on an issue-by-issue basis of SNP MPs to put together a majority, then he would be able to put together a majority.