Alistair Darling has accused the SNP of a U-turn on its plans to use the pound after independence.
The ex-Labour Chancellor seized on the party's suggestion it may not be able to secure a currency union with the rest of the UK after a Yes vote.
The SNP's Treasury spokesman, Stewart Hosie, raised the prospect of a stability pact instead.
But Mr Darling said that would leave Scots at the mercy of interest rates they had no say over.
An SNP spokesman said Mr Darling was "talking rubbish", adding that SNP policy was for an independent Scotland to be part of a formal sterling zone.
Earlier in the Commons Stewart Hosie, the party's Treasury spokesman, asked Scottish Secretary Michael Moore "while it makes far more sense to have a formal union, does he not agree that a stability pact based around debt and deficit levels is perfectly sensible?"
He added that such a move could "in no way be portrayed as a foreign currency running Scotland's economy".
Mr Darling said: "This is an astonishing U-turn from the nationalists.
"For months they have told us that we would be in a currency union but today they admit that we might not have our own currency after all."
l The Scottish Government has been asked whether its plan to siphon off money for a Norway-style oil fund will be funded by topslicing existing taxation or levying more taxes on the industry.
Speaking during an oil debate in Holyrood yesterday, Labour energy spokeswoman Rhoda Grant said: "It is unclear whether this will be topsliced from existing taxation, and if indeed the levels of taxation will remain comparable to the rest of the UK, or whether this will be a new tax levied on industry."
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