"Fiat money, in extremis, is accepted by nobody," Alan Greenspan, the former chairman of the US Federal Reserve, told lawmakers in Washington almost a decade ago. "Gold is always accepted," he added.

The "in extremis" scenario was for years only a possibility in the mind of die-hard gold bugs, but the financial crisis is leading regular investors - from the ultra-rich to middle-class savers - to believe that the environment in which Mr Greenspan said fiat money would be worthless is now around the corner.