ED Miliband has ruled out agreeing to a currency union with an independent Scotland if he becomes Prime Minister.
The Labour leader said it would be a manifesto commitment for next year's UK General Election.
He said the costs were too high and were more damaging than the transactions costs that UK businesses could be forced to pay without it.
Mr Miliband was speaking in Glasgow with Better Together leader Alistair Darling and Scottish Labour leader Johann Lamont.
Mr Miliband said: "I want to avoid transaction costs. The costs of being in currency union without the political, fiscal and banking union are way too high for the UK.
"We are not going to sign up to a currency union. It is not in the interest of the UK and I doubt it is in the interest of Scotland as well."
When asked if he would include it in the manifesto next year he said "yes".
Mr Miliband said the people who would suffer the most from independence and no currency union would be the people at the lower end of the income scale.
He said: "When economic instability hits, the better off are protected. Those at the top can move their money. It would be ordinary people left to bear the costs of higher interest rates and higher prices."
Mr Darling said the currency is central to the argument, and on the alternative to a currency union, he said: "There are six weeks to go and they don't know or won't tell us. It is not good enough."
He added: "No-one wants to join the euro or set up a new currency in turbulent economic times.
"Alex Salmond flirted with dollarisation (using the pound without a currency union) but that's the one his fiscal commission said not to touch with a barge pole."