He added: "The rest of the UK exports nearly £60bn year to Scotland - we are the rUK's second biggest export market.
"A separate currency would entail transaction costs of around £500m a year to rUK businesses.
"It is therefore clearly in the interests of the rest of the UK to share a currency with an independent Scotland.
"Scotland's huge contribution to a sterling area's balance of payments is another reason why it will be in the interests of the rUK."
Commenting for Better Together, Labour MP Ian Murray said: "This is another hugely experienced expert who has underlined the currency chaos which has plagued Alex Salmond's campaign.
"When a voice such as Douglas Flint's warns of the risks of money leaving a separate Scotland to somewhere safer, you pay attention.
"With just days to go until up to a million Scots start voting by post, Alex Salmond is running out of time to name Plan B.