It would not be practical for an independent Scotland to carry on using the pound as Scottish National Party leader Alex Salmond hopes, according to billionaire fund manager George Soros.
Salmond, who is leading the charge for the pro-independence campaign, wants Scotland to have a formal currency union with the UK even if it split from it.
But Chancellor George Osborne has said if Scotland leaves the UK then it will lose the pound. His opposite number on the Labour benches, Shadow Chancellor Ed Balls, has said the same.
"I don't think that Scotland leaving and becoming independent and yet remaining part of sterling and the Bank of England is actually practical," Soros said at a press conference in London.
Soros suggested joining the euro as an alternative for Scotland and stressed that an independent currency would be "very inefficient and potentially dangerous".
Adopting the euro would also require an independent Scotland to join the European Union (EU) – something European Commission President José Manuel Barroso said would be "extremely difficult, if not impossible".
Bank of England Governor Mark Carney said in a speech outlining the technicalities of a Scotland-UK currency union that strict fiscal rules would have to be established for it to work. This would mean Scotland giving up some of the sovereignty it desires.
Scots will vote in a September referendum on independence.