The Better Together leader again urged Alex Salmond to spell out his 'Plan B' for the currency a day after the First Minister announced the eagerly awaited blueprint for independence will be published on November 26.
The document will outline the SNP's plan to seek a currency union with the UK, allowing the two countries to share the pound.
But the anti-independence campaigner said yesterday the SNP could not "assume" the UK would agree to the so-called sterling zone.
Mr Darling said: "This is absolutely fundamental to Scotland's future and if he (Mr Salmond) can't answer the question definitively as to what the fall back position would be, then this White Paper will simply fall flat on its face. It will not be worth the paper it is written on. It is fundamental what currency we use and he can't answer that question."
He added: "No-one but a fool would go into a negotiation without a Plan B.
"So let's hear it. We want to hear it in this White Paper because if we don't then absolutely nothing he says will have any credibility whatsoever."
Chancellor George Osborne and shadow chancellor Ed Balls have warned the UK would be unlikely to agree a currency union, arguing it would expose taxpayers south of the Border to additional risks.
Stewart Hosie, the SNP's Treasury spokesman, said: "It was Mr Darling himself who said in a broadcast interview earlier this year that a sterling area between an independent Scotland and the rest of the UK would be 'desirable' and 'logical', which is indeed the case.
"If Mr Darling can't keep his story straight from one interview to another, no wonder the No campaign is losing the plot."