Speaking at a briefing organised by the pro-UK Better Together campaign, Professor Ronald MacDonald of Glasgow University, said Alex Salmond's proposal to share the pound after a Yes vote was doomed to fail because an "oil effect" would create a huge mismatch between the Scottish and UK economies.
He said the Scottish Government's "only tenable" alternative to the plan - which has been publicly ruled out by the main UK parties - would be to create a new currency.
However, he warned that could only be done if Scots were willing to accept years of austerity as the newly independent state built up foreign exchange reserves to underpin the currency and took action to reduce its deficit.
Professor MacDonald, one of the world's top experts on currency and a former adviser to the International Monetary Fund, estimated that an independent Scotland would have to cut spending by around £12 billion each year, equivalent to the annual cost of the NHS.
Like others around the world, he said, an independent Scotland would see rising prices and wages, making non-oil firms uncompetitive and leading to higher unemployment.
Without full political union, he said the two countries' diverging economies would place impossible strain on a currency union between countries with different inflation rates and economic priorities. "I don't think it would last long," he warned.
The inevitable currency "bust-up," he estimated, would cost the economy £30bn to £100bn, plunging the country into a deep recession lasting up to seven years.
Professor MacDonald also dismissed the fall-back option, hinted at by Mr Salmond, of keeping the pound without a formal agreement with the UK, known as sterlingisation.
He said the plan would require spending cuts of about £12bn each year, equivalent to the annual cost of the NHS, to bring the country back into the black or face serious financial instability. He said a new currency was "the only tenable Plan B".
However, establishing a new currency would require years of austerity to build up foreign exchange reserves to manage exchange rates.
Jim Gallagher, an adviser to Better Together, said: "No-one in Better Together is saying Scotland could not manage on its own.
"It is a question of whether it would manage better or worse. And to get to a stage where it could manage, you would have to make huge economic changes."
A spokesman for the First Minister dismissed the criticism. He said: "The Fiscal Commission Working Group's recommendation was entirely predicated on an independent Scotland's economy. It isn't anything other than a look forward. The opposition narrative seems to be that having oil is a dreadful curse for Scotland. That's a preposterous notion."