Joining the euro would be Alex Salmond's "only realistic plan B" for the currency of an independent Scotland, shadow chancellor Ed Balls has said.
The Labour MP, who opposed UK entry to the euro when he worked for Gordon Brown, has reiterated his warning that the SNP's preference for keeping the pound in a formal currency union with the remainder of the UK is "off the table".
He has pointedly ruled out a currency union if he becomes Chancellor, echoing the refusal of current Tory Chancellor George Osborne and Scottish Lib Dem Treasury Secretary Danny Alexander.
Keeping the pound without a currency union would accelerate "the movement of financial services out of Scotland", while a new currency would be "very expensive, very risky".
"I fear that an independent Scotland would end up finding that joining the euro would be the least worst of all the bad options," he told The Observer newspaper.
"It's not what I would choose for Scotland. And I am not surprised at all that Alex Salmond doesn't want to admit it now, but joining the euro would likely be his only realistic plan B."
He added: "Given the size of the UK relative to Scotland, given that Scotland would be leaving the UK, the size of the Scottish financial sector and given the risk, therefore, to UK taxpayers, a sterling currency area would be off the table.
"Attempting to use sterling as the currency of an independent nation without any say over the actual operation of that currency and Bank of England policy would put the Scottish economy in an impossible position, and it would accelerate what would happen anyway - the movement of financial services out of Scotland.
"There is no way you could have banks or insurance companies operating in Scotland when you don't have a lender of last resort, as monetary policy maker, to back the financial sector if you got into trouble.
"Scotland could attempt to go it alone with its own currency. That would be very expensive, very risky, and people would pay a big price in terms of higher interest rates for mortgages and loss of trade, because it would make the business environment very unstable and difficult."
Mr Balls urged Mr Salmond "to tell people what plan B is" at his next debate with Better Together leader Alistair Darling.
A spokesman for Mr Salmond told The Observer: "An independent Scotland will keep the pound because it's our currency, too, and pensions and public services will be more sustainable after a yes vote because Scotland's economy is stronger than the UK's."
He said: "The last three polls all show significant swings to yes, with support as high as 48%, and we are looking forward to the debate as an opportunity to communicate the positive message why Scotland can, should and must have the powers of an independent country."
A Better Together spokesman said: "This week up to one million Scots will receive their postal vote forms, yet we still don't know what currency we would use if we leave the UK. We need to know Alex Salmond's Plan B.
"Would we rush to adopt the euro or set up a separate unproven currency?
"This couldn't be more important. We need to know what money our wages, pensions and benefits would be paid in.
"How can we have confidence about the future if we don't know what currency we would use to pay our shopping and energy bills? We have no idea what money we would use to pay for our NHS if we left the UK.
"Alex Salmond wants us to vote for independence on the basis of his blind faith and guesswork alone. We should say no thanks to taking on so much risk."
SNP treasury spokesman Stewart Hosie said: "There are already over 230,000 Labour voters planning to vote Yes, and this ill-informed nonsense from Ed Balls helps explain why.
"It's embarrassing that Mr Balls - a man aspiring to be Chancellor of the Exchequer - seems unaware that there are entry criteria for joining the euro, and an independent Scotland wouldn't qualify because one of the prerequisites is membership of the exchange rate mechanism, which is itself voluntary - so there is no way Scotland could be forced to join the euro.
"The more Westminster politicians like Ed Balls try and pretend to people in Scotland that this immensely wealthy country is actually some kind of economic basket case, the more people move to Yes."