DAVID Cameron will today ruthlessly play the Scottish Nationalist card, warning voters in England that a Labour-SNP alliance would mean "back-room deals, bribes, ransom notes, chaos" at Westminster not for a week after the General Election but for five long years.

With just two days to go before polling day, the Prime Minister will focus his attention once more on the Liberal-Conservative marginals of the West Country and make a last-ditch attempt to get non-Conservative supporters to vote tactically to stop what he believes is the nightmare of an Ed Miliband-Nicola Sturgeon pact.

At a rally this afternoon, Mr Cameron will claim the SNP leader - who is "on the television all day, every day", telling people she wants to put Mr Miliband in Downing Street - is intent on raising taxes to pay for more welfare while Nick Clegg is "no better" and would happily support a Lab-SNP alliance.

"Back-room deals, bribes, ransom notes, chaos; not just for the week after the election but for five long years. It doesn't bear thinking of. Our defences weakened. Our ability to pay our way questioned. Our United Kingdom threatened. All of your work and sacrifice that has put this country back on its feet again completely wasted.

"And the consequences for you? Higher taxes to pay for the extra borrowing; higher mortgage rates caused by the instability; jobs lost because of the insecurity."

But the PM will argue there is an answer: vote Conservative. And in a direct appeal for non-Tory supporters to vote tactically to keep Mr Miliband out of Downing Street, he will add: "So I say to voters in LibDem seats - the only way to avoid Miliband/SNP and economic chaos is to vote Conservative this time."

Appealing to Ukip voters, he will say: "The only way to avoid Milliband and the SNP is to vote Conservative this time; Nigel Farage is the back-door to a Labour Government.

He will add: "And I say to previous Conservative voters, who may be thinking twice about turning out - turn out because if the nation is to avoid Miliband/SNP and economic chaos, you must vote."

As questions continue to be asked about what would be a legitimate government in the event of another hung parliament, Ms Sturgeon opened up the prospect of a bitter constitutional row after claiming a Conservative-led government, reliant solely on English MPs for support, would not be lawful.

At a rally in Dumfries, the First Minister raised the political stakes when she said it would not be acceptable for the UK to be governed by "the largest party in England" if the votes of Scots were not required to support it.

She argued it would be "more balanced" to have an alliance of parties in power representing the whole UK than "a party trying to take power without UK-wide support".

But a Scottish Conservative spokesman dismissed the idea, saying: "Whoever can command a majority of those members in the Commons will be able to form a government regardless of whether they come from Scotland, England, Wales or Northern Ireland. Nobody is being ignored; this is the parliamentary system."

Elsewhere, after Ed Miliband refused to say if Labour - having ruled out an arrangement with the SNP - could form a legitimate government with fewer seats than the Conservatives, party sources talked up the idea of forming a minority Coalition with the LibDems.

The thinking is that such a tie-up would be able to counter claims that a minority Labour administration lacked legitimacy if it had fewer seats than the Tories. A Lib-Lab minority government could out-vote the Tories whenever the SNP abstained.

Conservative HQ said: "This confirms that if you vote Liberal Democrat, you'll get an SNP-led Miliband government. This 'minority coalition' would be propped up by the SNP."

Lord Scriven claimed Mr Cameron had "taken to lying" after admitting privately to Mr Clegg that the Conservatives could not form a majority government.

Tory HQ insisted the claim was "100per cent untrue" while Mr Cameron said it showed the Deputy Prime Minister was becoming "increasingly desperate".

In Glasgow, comedian and Labour campaigner Eddie Izzard hit out at "violent" and "aggressive" Nationalist protesters as scuffles broke out when he campaigned in Scotland with Jim Murphy.