Iain Duncan Smith has accused the SNP of being in "complete denial" about the cost of welfare and pensions in an independent Scotland.
The Work and Pensions Secretary said that Scotland would be unable to afford its bills if it left the UK.
His comments came after an SNP MP accused his flagship plan to encourage millions off benefits of being a "complete fiasco".
Last week Mr Duncan Smith's department announced that it would not meet its 2017 deadline for the complete roll out of Universal Credit scheme.
SNP MP Eilidh Whiteford called on Mr Duncan Smith to suspend the roll-out until after the independence vote.
But the Cabinet minister told her: "All I would say, is they are in complete denial about the cost of welfare and pensions.
"They will not be able to afford the pensions bill with their demographic set-up.
"The welfare costs alone are going to cost them so much money.
"I don't know where they think they are going to get the money from if they break free from the UK."
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