SNP plans for a currency union with the rest of the UK if Scotland becomes independent are "increasingly dead in the water", Alistair Darling has said.
The leader of the pro-union Better Together campaign launched a further attack on the Scottish Government's white paper, which sets out its vision for independence, criticising the Nationalists' proposals on a range of issues including currency, universities, and debt and borrowing.
Speaking to an audience of young voters in Edinburgh, Mr Darling also argued that there was "no way" the European Union (EU) would allow an independent Scotland to charge tuition fees to students from south of the border.
He called on Mr Salmond to bring forward legal advice on this issue as well as produce a plan B for Scotland's currency arrangements.
The First Minister has already "damaged his credibility" over the Scottish Government's legal advice in relation to EU membership while the white paper is "largely a work of fiction" and a "catalogue of promises", the former chancellor said.
Addressing his audience on the SNP's proposed currency union, which would enable it to keep the pound, Mr Darling said: "Does he not accept that a currency union needs both Scotland and the rest of the UK to agree to enter into it?
"Should he not accept a currency union means both sides have to agree each other's budgets?
"That is what is happening in Europe. And that is why it is difficult to see anyone signing up to it. That is why it is increasingly dead in the water."
Speaking afterwards, Mr Darling went on to say it was difficult to see why even independence supporters would back a currency union, likening the idea to a creation by illustrator Heath Robinson, known for his drawings of complicated machines.
He said: "All European Union members have to submit their budgets for approval to the European Commission. I think it's profoundly anti-democratic.
"It means if Scotland and the rest of the UK entered into a union, you'd have to agree each other's tax and spending. You can see it across the channel as we speak.
"I just don't think it is going to happen. If Alex Salmond insists there is a glimmer of hope here, given he can't deliver it as it takes two to work, he has got to have a fallback position.
"What he is proposing is a real Heath Robinson affair. If I was a nationalist, I couldn't even see why I would sign up for it."
SNP Westminster leader Angus Robertson described Mr Darling's speech as "poor and panicky".
He said a poll, commissioned by the SNP, showed support for the continued use of the pound in the event of independence.
When asked do you think that Scotland and the rest of the UK should continue using the pound in an agreed sterling area, 71% of over 1,000 people surveyed in December by the polling firm Panelbase answered yes.
Mr Robertson said: "Mr Darling's own No campaign expert adviser, Professor Gallagher, agrees that Scotland will not be part of the euro - something else he shows no indication of being aware of.
"Given that the Westminster Treasury has just published detailed proposals agreeing with the Scottish Government policy to share the UK's debt - much of it run up by Alistair Darling - we presume that they wish Scotland to pay a share in pounds sterling."
He added: "The Scottish Government has published a detailed, 670-page guide to an independent Scotland - covering such matters as currency and student funding - and it is high time that Alistair Darling answered the 50 questions posed by Nicola Sturgeon about the dangers and downsides of voting No.
"There are no new ideas or messages from the No campaign - only further evidence that Project Fear is alive and well in 2014."
Mr Robertson described Mr Darling as "out of touch", stating that "Yes moves into the lead when people hear the positive arguments for an independent Scotland, and they are seeing right through the smears and fears of the No campaign".
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