The SNP has outlined what it says are "perfectly sensible" plans that could block a Labour minority government's spending on defence as part of a row over Trident.
Deputy leader Stewart Hosie said the party would be prepared to vote against giving ministers the authority to spend money.
The Conservatives accused the party of planning to hold ministers to ransom and US-style 'gridlock' that could see soldiers not paid their wages.
The SNP have said Trident is a red line for any 'informal coalition' - or confidence and supply - deal with Labour after May.
But if Labour went it alone as minority government, Mr Hosie said that his party would vote against "cuts that we didn't like, and the spending would appear in the estimates and we would of course vote against the spending we didn't want to see."
"In the absence of a five-year deal, there would be no deal, so we would be perfectly at liberty to table amendments to Budgets, table amendments to legislation, vote against or table amendments to estimates. Perfectly sensible," he said.
Mr Hosie denied that blocking parts of government spending could topple a Labour minority administration.
Conservative chairman Grant Shapps said: "The SNP are threatening to hold Britain to ransom to guarantee an Ed Miliband government gives them what they want - weaker defences, more borrowing, more debt and more taxes.
"If the SNP vetoed defence spending for a year, our troops risk not being paid. Working people across our country will pay dearly for the economic chaos of Labour-SNP."
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