First Minister Nicola Sturgeon has explicitly threatened that the SNP could vote down a Labour budget.
Ms Sturgeon told the BBC that SNP MPs would vote against a Labour budget that sought to impose cuts opposed by her party. However, she said the SNP would not “sink” the government.
Her comments came as Labour leader Ed Miliband insisted his rejection of a deal with the SNP is on “principle” and he would “never put the Tories into government”.
The Labour leader has come in for heavy criticism after ruling out any pact with the Nationalists after next week’s election, even if it meant his party fails to form a government.
However, during a passionate rally in Glasgow before which flag-waving protesters aggressively confronted attendees, shouting “Red Tory scum” and screaming through megaphones Mr Miliband said Labour stood for solidarity while the SNP wanted separation.
To a noisy standing ovation, he said: “I will never put the Tories into government. I have spent my entire political career fighting them. But the tragedy is that the SNP may very well let the Tories in. That’s what could happen if the Tories are the largest party.
“But I am also clear there will be no deal, no pact, no coalition, no tie-in with the SNP; and I don’t say that for tactical reasons I’m advocating this for principled reasons.”
Scottish Labour deputy leader Kezia Dugdale said later: “Many people who have trusted Nicola Sturgeon must be shocked to hear her say that she will cut down a Labour budget,” she said.