A detailed assessment by Citigroup issued yesterday said it was "astonishing" the Scottish Government had not outlined an alternative currency plan after UK ministers ruled out proposals to create a monetary union.
Analysts said an independent Scotland's prospects were unclear - largely as a result of uncertainty over the currency - but warned of higher borrowing costs.
The comments came as Chief Secretary to the Treasury Danny Alexander said the UK Govern-ment's opposition to the currency was "final".
In a separate blow to the Yes campaign, Dundee investment house Alliance Trust became the latest business to warn shareholders over the possible consequences of independence.
The 125-year-old firm said it was registering new companies in England as a precaution.
In response, the Scottish Government again insisted its "sensible proposals" for a currency union would be accepted in the event of a Yes vote as First Minister Alex Salmond chaired a meeting of his Council of Econo-mic Advisers, including business-man Jim McColl, at Bute House in Edinburgh.
In its analysis, Citi Research, an arm of the global bank, said: "We regard a sterling monetary union as unlikely but we are genuinely unsure what currency and monetary policy would be adopted by an independent Scotland.
"In our view it is astonishing the Scottish Government, in seek-ing independence, has reached this stage: seeking a currency union without agreement with the rest of the UK and without a clear alternative plan."
The report highlighted Scot-land's high fiscal deficit, falling oil revenues, the size of the banking sector compared with the rest of the economy and uncertainty over the currency as "concerns".
It warned of higher borrowing costs if an independent Scotland used the pound without agree-ment and back-up from the Bank of England.
However analysts said they did not regard threats from the Scottish Government to refuse to take a share of the UK's national debt, if a currency union was blocked, as "wholly a bluff".
They said the move - again raised by Finance Secretary John Swinney yesterday - would push up the cost of borrowing but should be taken seriously because of the scale of Scotland's debts.
Uncertainty over the currency was among a list of concerns raised by Alliance Trust. The company, which reported to shareholders yesterday, also cited regulation and tax on savings and pensions as possible risks.
In a statement chief executive Katherine Garrett-Cox said registering new companies in England would give the business "flexibility".
She said: "The referendum in September is creating uncertainty for our customers and our business, which we have a responsibility to address."
A number of firms, including generator giant Aggreko, Lloyds Banking Group, Barclays, Standard Life and Royal Bank of Scotland, have also listed indep-endence in risk management sections of annual reports in recent days.
Secretary of State for Scotland Alistair Carmichael said: "It's clear independence would be a big problem for Scotland rather than the solution."
Mr Swinney said: "The issues raised by Alliance Trust are entirely addressed by the propositions put forward by the Scottish Government, and show exactly why our proposals for a formal currency area are the right proposals, why they are in the best interests of business on both sides of the border and why that is what will be implemented by both governments."