The poorest people stand to lose the most if an independent Scotland loses the pound, Labour leader Ed Miliband said.
He confirmed that he could not agree to a "eurozone-style currency union" if Scotland votes Yes to independence and he becomes UK prime minister in May.
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However, the SNP has challenged Mr Miliband to explain how he would head off the cuts to Scotland's budget that would flow from the "privatisation of the NHS" in England.
Every £10 cut from NHS England would result in a £1 cut in Scotland's health budget, Scottish Health Secretary Alex Neil said.
Speaking ahead of an address to the Scottish Chambers of Commerce in Glasgow, Mr Miliband said: "On Tuesday night Alex Salmond didn't have an answer on the pound.
"The currency that Scotland uses is crucial for Scotland's future.
"Nobody claiming to be a social democrat who cares about Scottish pensioners, Scottish families and Scottish businesses should dare take this risk without a currency plan.
"If you care about social justice in our country, you can't leave the economics to guesswork.
"That's why businesses and families are demanding answers on the currency.
"Those at the top can move their money across the border and keep the pound.
"We know that the people who would stand to lose most from making this decision are those who have the least.
"It would be working people, small businesses across Scotland who would be left to deal with the consequences of no Plan B."
He added: "I want business and families to have the certainty of the pound. But I can't support a eurozone-style currency union.
"We did not join the eurozone for clear and correct economic reasons.
"It is for the same reasons the rest of the UK should not enter into a currency union with an independent Scotland.
"And that's why, as prime minister, I couldn't agree to a currency union."
He confirmed Labour would strengthen powers over tax, welfare and jobs.
Mr Neil said: "Mr Miliband knows perfectly well how Scotland is funded under devolution - now he must come clean on the huge impact that NHS privatisation in England will have on this budget.
"It was of course Labour who opened the floodgates on privatisation when they were last in office.
"Now Ed Miliband and his colleagues are going round telling everyone that privatisation will eventually destroy everything that is precious about the NHS.
"Does he seriously expect the people of Scotland to believe that the objective of this privatisation is to increase public spending?
"Cuts to our NHS budget as a result of Westminster privatisation is a risk Scotland doesn't need to take.
"We have protected the NHS budget in real terms but only a Yes vote gives us the opportunity use Scotland's immense wealth to ensure our public services are properly funded in the long-term."
Finance Minister John Swinney said he believes the UK government will agree to a currency union if there is a Yes vote.
He told BBC Radio Scotland: "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, and that to me is a ridiculous proposition from the other parties.
"They cannot turn round to the electorate in the rest of the UK and say we're going to let the Scots go away debt free from the United Kingdom, we're going to let them off of an annual cost of £5 billion, that's why they will agree to a currency union after independence."