THE legitimacy of a minority Labour government supported by the SNP has been called into question by Conservative high command as sources said David Cameron would attempt to stay on in Downing Street even if he did not command a Commons majority.

In 2010, Gordon Brown clung onto power for a few days before it became clear the Liberal Democrats were intent on doing a deal not with Labour but with the Conservatives.

Talk of a looming constitutional crisis came as once again as Mr Cameron's party emphasised the threat they say Nicola Sturgeon and her colleagues would pose to the cohesion of the UK.

The Prime Minister branded Nationalist economic policy "bonkers" and William Hague, the ex-Foreign Secretary, claimed the SNP holding the balance of power would "try to divide Scotland against England with everything they did every day".

But the First Minister hit back, claiming Ed Miliband had been "spooked" by anti-SNP rhetoric and that he had allowed himself to be bullied by the Tories into making his "disastrous" announcement on not doing a deal with the Nationalists.

"It was a cack-handed move," she insisted, "because in Scotland it simply galvanises even more support for the SNP."

Last month, Theresa May, the Home Secretary, warned that a Lab-SNP alliance would spark the greatest constitutional crisis since the abdication of Edward VIII and that such a scenario would "raise difficult questions of legitimacy" as many English voters would be unhappy with a government being run by a party they could not vote for and who wanted to break up the Union.

She again pointed to problems with a Lab-SNP alliance in power, saying: "In constitutional terms, the shots would be called by somebody who wouldn't even be in parliament," ie Ms Sturgeon.

Under the Fixed Term Parliaments Act, the incumbent premier has first crack at forming a government; if he or she cannot, then it falls to the Leader of the Opposition to try to form a majority to get through their programme of government in a Queen's Speech.

But senior Tories have suggested Mr Cameron would claim victory if the Tories were the largest party, suggesting that this, in the eyes of the public, would mean he would have legitimacy and should form a new government. In these circumstances, the Conservative leader would seek to put Mr Miliband on the spot, claiming he would need to do a deal with the SNP to form his own government; a possibility the Labour leader ruled out of hand last week.

Three senior Tories said to have the ear of the PM made clear that Mr Cameron would quickly argue that Labour could not claim "legitimacy" to form a government if it was behind the Conservatives in terms of seats and votes and needed Nationalist backing.

Meantime, it was suggested senior members of Mr Miliband's team had sought legal advice over whether it would be possible to force Mr Cameron out of office with an immediate vote of no confidence.

The aim would be to deny the Tory leader the chance to build support from minor parties before announcing his Queen's Speech, which is not scheduled to take place until May 27, three weeks after polling day.

On the stump, the PM continued to ramp up the anti-SNP rhetoric, claiming that in any tie-up with Labour, Ms Sturgeon would "eat Miliband for breakfast, lunch and tea".

In a campaign speech, he said a Lab-SNP alliance would be "the road to ruin" and "a calamity for our country".

Hoping to capitalise on his relatively strong personal ratings south of the border, Mr Cameron asked those thinking of voting Ukip or Lib Dem to switch to the Tories in order to keep him in No 10.

"If you have got a view on who you would prefer as your Prime Minister express it at the ballot box," he declared.

"The outcome will not be decided any other way. Do not risk voting for another party and hoping that is the outcome. If you want your preferred Prime Minister get out there and vote for it."

He warned that Ukip was a "back door to a Labour Government" and suggested Nick Clegg was ready to be part of a "minority Labour Government, propped up by the SNP".

Elsewhere, Boris Johnson, the London mayor also raised fears about a Lab-SNP tie-up, branding it "Ajockalypse Now!" He said it would lead to government in "a fundamental state of paralysis and permanent civil war".

The Conservatives used their latest election broadcast to press home dire warnings about the threat of SNP influence. In the film, the economy, represented by a clock, is smashed by a sledgehammer that represented the Nationalists.