Former chancellor Alistair Darling, who also leads the Better Together campaign to keep Scotland in the UK, said Bank of England governor Mark Carney's speech on currency union spells out stark problems.
Mr Darling said: "This is a detailed speech but make no mistake - the governor's judgment on currency unions is devastating for Alex Salmond's currency plans. Why? Because the whole point of independence is to break the fiscal and political union that makes monetary union possible.
"The governor has spelled out in stark terms the problems of a currency union. Above all, it needs people living in the rest of the UK to agree to something they have never been asked about.
"As the governor makes clear, in a currency union both sides have to agree to each other's taxes, spending and borrowing. This is what is happening in the eurozone today.
"It is highly unlikely that the people living in the rest of the UK would agree to this. And remember, in a currency union like this, Scotland has 10% of GDP and the rest of the UK would have 90%. It is clear who would call the shots."
Economics Editor Richard Edgar is in Edinburgh and listened to the Bank of England governor's speech on how a newly-independent Scotland could retain sterling:
Treasury's blunt response to Carney's speech: "A currency union is highly unlikely to be agreed. The Scottish Government needs a Plan B.”
Economics Editor Richard Edgar is in Edinburgh for the Bank of England Governor's speech on how a newly-independent Scotland could still retain sterling:
Key message from Carney: "a durable, successful currency union requires some ceding of national sovereignty."
Tough decisions on giving up sovereignty to be made both by Scots ... and rest of UK.
First Minister Alex Salmond said he had an "excellent meeting" with Bank of England governor Mark Carney over Scottish independence.
Mr Salmond, who wants a currency union with the UK if Scotland backs independence in a referendum in September, also reiterated that "the Bank of England is independent and doesn't intervene in politics."
Bank of England governor Mark Carney's decision to speak publicly about the possible consequences of Scottish independence has been welcomed by Downing Street.
Prime Minister David Cameron's official spokesman said: "It's hardly very surprising that the independent governor of the Bank of England might wish to address and consider some of the issues that are involved.
"The issue around currency is an important part of the debate that is currently going on in Scotland. It hardly seems a great surprise at all, on the technical issues, that the governor of the Bank of England might want to set out his views.
"I'm sure the people of Scotland will want to be as well-informed as possible."
The Bank of England Governor has warned that a newly-independent Scotland would be forced to hand over some national sovereignty if it wanted to keep the pound.
But Mark Carney appeared to step back from getting involved in the political row oer independence, saying: "Decisions that cede sovereignty and limit autonomy are rightly choices for elected governments and involve considerations beyond mere economics.
"For those considerations, others are better placed to comment."
Scotland's First Minster has said that Mr Carney's predecessor at the Bank of England, Sir Mervyn KIng, had suggested the Treasury could change its approach in the event of a Yes vote.
Alex Salmond told how he had met Sir Mervyn "a couple of years back", and added: "The first thing he said to me was 'your problem is what they say now', meaning the Treasury, 'and what they say the day after a Yes vote in the referendum are two entirely different things'."
Mr Carney, who took over as Governor of the Bank of England in July, has already warned of "challenges" of adopting a shared currency without having "certain institutional structures" put in place.
First Minister Alex Salmond has confirmed that talks with the Bank of England governor Mark Carney over a currency union if Scotland wins independence will continue in the run-up to the referendum.
Mr Salmond held talks with Mr Carney in a private meeting today, but the First Minister said afterwards: "I was delighted to welcome the new Bank of England governor to Edinburgh on his first official visit to Scotland since his appointment.
"We greatly value our strong working relationship with the Bank of England and its commitment to operational independence and impartiality in political debate.
"The discussion was private but I welcome that the governor has confirmed his willingness to continue technical discussions, inaugurated by his predecessor Lord King, between the Scottish Government and the Bank of England in advance of the referendum."