The UK Government has moved to distance itself from claims made by an anonymous minister that there would be a currency union between an independent Scotland and the rest of the UK.
Downing Street insisted that independence means "walking out of the UK pound" but Deputy First Minister Nicola Sturgeon said the minister's comments had deprived the No campaign of its "trump card".
The unnamed minister told The Guardian that "of course" there would be a decision to share the pound, indicating that a deal could be done with Scotland in exchange for the UK's nuclear submarine fleet remaining at Faslane.
All three main parties at Westminster have officially ruled out sharing sterling with an independent Scotland, but the minister told newspaper: "Of course there would be a currency union."
The minister added: "There would be a highly complex set of negotiations after a yes vote with many moving pieces. The UK wants to keep Trident nuclear weapons at Faslane and the Scottish government wants a currency union - you can see the outlines of a deal."
The newspaper reported that the minister is at the heart of the pro-union campaign and had acknowledged that opposition to sharing the pound was a vital part of the strategy.
The minister said: "You simply cannot imagine Westminster abandoning the people of Scotland. Saying no to a currency union is obviously a vital part of the no campaign. But everything would change in the negotiations if there were a yes vote."
But a No 10 spokesman insisted: "There will not be a currency union in the event of independence. The only way to keep the UK pound is to stay in the UK. Walking out of the UK means walking out of the UK pound.
"A currency union will not work because it would not be in Scotland's interests and would not be in the UK's interests.
"Scotland would have no control over mortgage rates, and would be binding its hands on tax and funding for vital public services.
"The Scottish Government are proposing to divorce the rest of the UK but want to keep the joint bank account and credit card.
"The UK would not put its taxpayers at risk of bailing out a foreign country and its banks. Parliament wouldn't pass it, and the people wouldn't accept it.
"Any suggestion to the contrary is wrong."
Scottish Secretary Alistair Carmichael said: "An anonymous, off the record quote does not change the stark reality on the currency.
"The UK Government has listened to the views of the Governor of the Bank of England and the independent advice of the Permanent Secretary to the Treasury that a currency would be damaging for all the United Kingdom.
"That's why a currency union simply will not happen.
"The Scottish Government should remove the uncertainty on the currency by coming forward with a plan B."
The minister's comments were seized on by Deputy First Minister Nicola Sturgeon, who said they gave a big boost to the Yes campaign.
She said: "This was supposed to be the No campaign's trump card, but as the polls show it has backfired badly - the gap between Yes and No has halved since November, and most Scots simply do not believe the bluff and bluster we had from George Osborne, Ed Balls and Danny Alexander.
"Now that the card has been withdrawn, it gives an even bigger boost to the Yes campaign. And it can only add to the sense of crisis which is engulfing the No campaign.
"The reality is that a currency union is every bit as much in the interests of the rest of the UK as an independent Scotland, and that is why Westminster will agree to one. Scotland is the rest of the UK's second biggest trading partner, and not sharing sterling would cost businesses south of the border an extra £500 million in transaction costs."
She added: "The leader of the No campaign, Alistair Darling, said that a shared sterling area between an independent Scotland and the rest of the UK is 'desirable' and 'logical' - and now the UK Government appears to have U-turned and actually agrees with that sensible position."
The Scottish Government's White Paper on independence, published last November, says a shared currency is in the "economic interests" of Scotland and the rest of the UK.
Almost half of voters north of the border think that Mr Osborne's pledge to rule out a formal currency union with an independent Scotland is a bluff, according to a poll published this week.
A YouGov poll for The Times newspaper suggests 45 per cent of Scots do not believe the Chancellor's threat is real, compared with 40 per cent who think he means what he says.