The risks to the UK of sharing the pound with an independent Scotland would outweigh the benefits of cutting the UK national debt or preventing cross-border transaction costs, Labour leader Ed Miliband has said.
Scotland's share of the UK debt would amount to about £120 billion - around £5 billion a year in repayments - but the Scottish Government has threatened to withhold its share if the UK does not agree to share the pound.
Scottish Finance Secretary John Swinney insisted the UK's cross-party refusal to consider a currency union "will crumble after a Yes vote" in the face of a higher UK debt, higher costs to UK businesses to trade with Scotland and a potential weakening of the pound due to the loss of Scottish oil and whisky.
But Mr Miliband said only 10% of UK trade goes to Scotland and pointed out that the 40% of UK trade that goes to Europe has not compelled Britain to join the euro.
Former Labour chancellor Alistair Darling said the UK Treasury would be "phlegmatic" about paying Scotland's share of the debt if there was a refusal.
Mr Swinney, speaking on BBC Radio Scotland, said: "The position will crumble after a Yes vote in the referendum because what Mr Miliband or Mr Cameron or Mr Clegg or whoever is leading the United Kingdom after the 2015 election would have to do would be they would have to say to the rest of the UK 'Look, we're going to put a barrier up for your trade with Scotland which would increase the cost for business in the rest of the UK by £500 million, we're going to undermine sterling by rejecting the contribution made by oil and gas and whisky into a sterling area, and we're also going to absolve the Scots of a £120 billion share of the UK debt which translates into an annual cost of £5 billion a year, simply because we won't agree to a currency union'.
"That, to me, is a ridiculous proposition from the other parties."
Speaking after an address to the Scottish Chambers of Commerce in Glasgow, Mr Miliband said: "Forty per cent of the UK's trade is with the euro area and 10% of the rest of the UK's trade is with Scotland.
"So, we're not about to join the euro because it's 40%, so therefore you have got to make a judgment about whether the currency union makes sense or doesn't make sense.
"Of course you want to avoid the transaction costs, which is one of the reasons we want the UK to stay together.
"But I'm afraid the cost of a currency union without political union, without a fiscal union, without banking union, are costs that will be too high for the UK."
Mr Darling said: "Having been the chancellor, I would say if you have got a budget of about £700 billion, you have to be sort of phlegmatic about £5 billion.
"But the bigger question you would have to ask yourself is, would it be in the interests of either Scotland or the rest of the UK to enter into this arrangement?
"If you look at it from a Scottish point of view, which is how I am looking at it, why would you want to get yourself into a situation where to be in a currency union you would have to reach an agreement on your budget?
"That's what they do in the eurozone, on your tax, your spending and your borrowing.
"And in that you would be in a relationship where you would have just under 10% of the economy and the rest of the UK had just under 90% of it.
"So, it would be a very unequal relationship, and yet you would find yourself locked in, by law, to terms and conditions over your tax and spending over which Scottish people have no say.
"Now, if I was a nationalist I would say no, which is why (former SNP deputy leader) Jim Sillars calls it stupidity on stilts and so does (Yes Scotland chairman) Dennis Canavan."
Mr Miliband's appearance in Scotland comes after Alex Salmond faced renewed pressure to reveal what arrangements would be put in place if a formal currency union with the rest of the UK cannot be negotiated.
He was challenged on the issue by all three Scottish party opposition leaders during First Minister's Questions at Holyrood, while Mr Darling called for him to announce his Plan B during Tuesday night's television debate.
Meanwhile, former SNP leader Gordon Wilson called on Mr Salmond to provide voters with more information on currency options, stating that "bold assertions are not enough".
He said: "The common-sense default position is that Scotland would continue to use the pound sterling, which is an international convertible currency.
"London could do nothing to stop Scotland from doing so. It is surprising Alex did not spell this out.
"He does not have to abandon his first preference of using the pound. He needs to explain the options simply."
SNP Treasury spokesman Stewart Hosie said: "The No campaign's scaremongering on the pound is crumbling."
He said Mr Miliband had " exposed the fundamental flaw in the No campaign's argument which is that they cannot say no to continuing to share our pound without saying that they would make voters in the rest of the UK pay debts that would otherwise be paid by an independent Scotland".
"I'm not hearing Ed Miliband or anyone else making that statement in London," he added.
"With a Yes vote we can use the economic powers of independence to further grow our financial sector and attract new business to Scotland - and to have an economic policy tailored to Scotland's needs, rather than London and the South East."