Two leading figures from the banking industry have hit out at Scotland's First Minister following his insistence that Westminster will agree to a deal to allow an independent Scotland to continue to use the pound.
Chancellor George Osborne and his Labour and Liberal Democrat counterparts have already stated that they would not agree to such an arrangement - but this has been dismissed as "bluff and bluster" by the SNP leader.
Now, Sir Martin Jacomb, the former chairman of Prudential, and Sir Andrew Large, a former deputy governor of the Bank of England, have said that a currency union is "not compatible with Scotland being politically independent and is therefore not on offer".
In an article in the Times, they said: "On the future currency people are understandably puzzled.
"Alex Salmond claims that nothing much will change, that threats otherwise are a bluff and that Scotland would keep the pound sterling - but although Scotland could keep using the pound, to promise 'no change' is a huge deception: the consequences would be enormous."
A spokesman for Scottish Finance Secretary John Swinney said a formal currency union would be "in the overwhelming economic interests of the rest of the UK".
But Sir Martin and Sir Andrew argued that when countries want both political independence and a single currency, "things can go awry".
The two men stated: "The best way to keep the pound would be through a currency union like today.
"That is what Mr Salmond says he wants, but it is not compatible with Scotland being politically independent and is therefore not on offer."
Sir Martin and Sir Andrew said it was "unsurprising" that the main Westminster parties had ruled out a currency union with an independent Scotland, insisting: "This is not an option."
They also said that while Scotland could continue to use the pound without any formal agreement from the UK following a Yes vote in September, this could present difficulties.
"Scotland would have no influence over sterling interest rates or the exchange rate. Nor could it rely on UK central bank support in hard times," they said.
They went on to warn that in these circumstances continuing to issue Scottish bank notes "could be difficult", adding: "Today's system works because people regard these as equivalent to Bank of England notes, but after independence the Bank of England would be in a foreign country."
While they said setting up a new currency "may compromise Scottish independence least and be Mr Salmond's best option", Sir Martin and Sir Andrew added that Mr Salmond did not support this.
"Given the enormous risks, it's not hard to see why," they said.
"Wouldn't today's currency union as part of the UK be the best way forward? We firmly believe it would."
Mr Swinney's spokesman said: "As part of the United Kingdom, economic wealth is drawn towards London and the South East; however, the powers of independence would enable Scotland to rebalance the economy and give Scotland a competitive edge.
"Scotland is one of the wealthiest countries in the world, more prosperous per head than France, Japan and the UK itself, but we need the powers of independence to make the most of our huge resources.
"The Expert Fiscal Commission Working Group, which includes two Nobel laureate economists, has already looked at different currency options and concluded that an independent Scotland should keep the pound, which is as much Scotland's as it is the rest of the UK's, a position accepted by a UK Government minister who told the Guardian that 'of course there will be a currency union'."
He added: "The comments on Scottish notes are simply wrong as by law every Scottish note is backed by deposits held by the Bank of England and would continue to be issued on the same basis.
"A formal currency union is in the overwhelming economic interests of the rest of the UK."
Scottish Labour MP and shadow business minister Ian Murray said the comments from Sir Martin and Sir Andrew were "significant interventions from senior experts and further undermine the nationalists' economic case for separation".
Mr Murray said: "We find ourselves in a ludicrous situation. We are being asked to trust that Alex Salmond is always right and everyone else is always wrong.
"How can we possibly vote to leave the UK when we don't even know what currency we would use if we did?
"Scots deserve to know the facts. What is Alex Salmond's plan B for what would replace the pound if we leave the UK? If it's a separate currency or joining the EU, then we deserve to know before going to the polls.
"It's very clear that a vote to leave the UK is a vote to lose the security of the UK pound.
"This would put at risk thousands of jobs in Scotland and push up costs for families here."
Scottish Conservative enterprise spokesman Murdo Fraser said: "The SNP is just desperately hoping people will either not hear these warnings or do as it does and blithely ignore them.
"But the reality is an independent Scotland would be left with three options on currency, all of which are extremely poor.
"The currency union will not happen, that's clear, and using the pound without the agreement of the rest of the UK will have unimaginable disadvantages."
He added: "The most sensible and secure way to proceed is to keep the pound, keep our currency union, and the only way to do that is through a No vote in September."