A Yes vote in next month's Scottish referendum risks "capital flight" from the country leaving its financial system in a "parlous state", the boss of HSBC has warned.
The bank's chairman Douglas Flint, who describes himself as an exiled Scot, said the sterling currency union was an "anchor of financial stability" for Scotland.
And the move to any of the alternatives would be "complex and fraught with danger".
The three main parties at Westminster have all ruled out the Scottish Government's preferred option of a formal currency union between an independent Scotland and the rest of the UK.
Mr Flint said this was "wholly consistent" with the approach taken in the wake of the financial crash and eurozone crisis.
The SNP wants a currency union between an independent Scotland and the UK
Writing in a personal capacity in the Daily Telegraph, he said: "The alternatives to a currency union include a completely independent currency, passive acceptance of a monetary policy designed in London for the rest of the UK, or, assuming Scotland rejoins the European Union, eventual membership of the euro.
"In all these circumstances, the transition from the existing currency union would be complex and fraught with danger.
"At the extreme, uncertainty over the Scotland's currency arrangements could prompt capital flight from the country, leaving its financial system in a parlous state.
"This could, in turn, place enormous pressure on Scotland's future fiscal policies.
"Scotland would give up the benefits of being part of a larger fiscal union with the stability that offers in terms of scale, diversification and fiscal transfers."
The Proclaimers are backing the independence cause
Mr Flint said Scotland would face "an enormous challenge" to introduce its own currency, while an informal use of the pound, so-called "sterlingisation", would put "enormous pressure" on fiscal policy.
He said: "Monetary policy itself would be imported from the rest of the UK; Scotland would be faced with monetary policy implementation without representation - a very odd form of independence."
Referring to the Westminster parties ruling out a formal currency union, Mr Flint said: "That decision is wholly consistent with the actions that have been taken in the aftermath of the financial crisis to minimise the risks to UK taxpayers from financial sector shocks arising in overseas and wholesale banking operations.
"It is also consistent with the knowledge gained from recent events in the eurozone, which have highlighted the challenges inherent in managing a currency union without political and fiscal union."
Former prime minister Gordon Brown has also poured scorn on Mr Salmond's plans for a currency union after independence arguing it would put Scotland in a "neo-colonial" position, with key decisions over matters such as interest rates made elsewhere.
Meanwhile, those campaigning for a Yes vote marked a milestone having collected one million signatures in support of independence.
Among those to sign were singing duo Craig and Charlie Reid, whose hits as The Proclaimers include I'm Gonna Be (500 Miles), I'm On My Way and Letter from America.
Charlie said: "A Yes vote is vital to secure the kind of society that we want."
Craig said: "This vote is very serious and I think if you have an opinion you should say it."