Alex Salmond should set out an alternative currency plan for an independent Scotland as "a matter of great urgency", Chief Secretary to the Treasury Danny Alexander has said.
Mr Alexander has written to Scotland's First Minister urging him to provide details of a new plan after all three of the UK's main political parties rejected the prospect of sharing the pound in a formal currency union.
He wrote that "a failure to set out an alternative will deny our fellow citizens the information they need to make a rational decision based on facts and evidence".
Mr Alexander's letter continued: "The confidence of many people and of much of the business community will be shaken if it transpires that no such alternative plans have been drawn up with the referendum now only a few months away."
Both the Liberal Democrats and Labour echoed the position on currency set out by Chancellor George Osborne.
Mr Osborne said: ''The SNP says that if Scotland becomes independent, there will be a currency union and Scotland will share the pound.
''People need to know that is not going to happen.
''Because sharing the pound is not in the interests of either the people of Scotland or the rest of the UK.''
He dismissed the unionist parties' threat as "bluff, bluster and bullying", insisting Mr Osborne did not speak for the people or businesses of the UK and casting doubt on the independence and credentials of civil servant who said a currency union would be "fraught with difficulty".
"There's a range of currency options for Scotland. The best one we think for Scotland, and indeed for the rest of the UK, is the sterling zone," he said.
Mr Salmond cast doubt on the independence and financial record of Treasury civil servants, questioning the record of Sir Nicholas Macpherson.
"Treasury civil servants do what they are told by George Osborne," he said.
"At one stage he actually questions the fiscal responsibility of the Scottish Government.
"In his tenure of office since 2005, under three successive Chancellors, he's authorised borrowing of £740 billion.
"Over roughly the same period John Swinney as finance secretary in Scotland hasn't borrowed anything.
"I'm afraid we are in a situation where the UK government departments and civil servants do what they are told by their political masters."
He added: "I think we have to articulate the plan A.
"I think we have to explain why it's in the interests and we can dismantle Osborne's argument's against it."