An independent Scotland would rely too heavily on North Sea oil, according to the man heading the campaign to keep the country in the UK.
Former chancellor Alistair Darling, who is chairman of Better Together, described the resource as a "tremendously volatile diminishing asset", which makes up between 10%-20% of Scotland's GDP.
He also claimed that not all decisions about North Sea oil were taken in Scotland, meaning its position differed from that of Norway. The SNP hopes to set up a fund similar to the Scandinavian country, where a portion of its oil revenue is put aside for the future.
Speaking at an event in Glasgow on Thursday night, Mr Darling said: "I do not argue that Scotland could not go it alone - that's a silly argument. But we would be rather too dependent, for my liking, on North Sea oil. It's worth between about 1%-2% of the UK's GDP, but about 10%-20% of Scotland's, and the trouble with oil is that it's a tremendously volatile diminishing asset."
"The Nationalists' argument is that, in the meantime, we have got all sorts of renewables, but the problem with that is it only works when the wind blows and at the moment it is totally dependent on the UK subsidy to make the sums add up. That's not a happy position to be in. The decisions about how much is extracted are not taken in Scotland, they're taken in Texas as much as anywhere else. The oil companies decide whether to spend money in the North Sea or in Russia or Nigeria or wherever."
The Labour MP also said the UK's future in the EU if Scotland left remained uncertain. Mr Darling said the EU favours large states but the future of both parts of the UK would be up for negotiation in the event of a Yes vote.
The SNP argues that both parts of the UK will be in the same position, favouring the view that both will be joint successor states but arguing that if Scotland is ejected the remainder of the UK will be too.
"No existing member state has ever seen any part of it separate, so there's no rules for leaving Europe because nobody ever imagined why anyone would want to," Mr Darling said.
"I think the decision will be taken on a political basis rather than a legal basis because there is no legal treaty with this eventuality. It all depends on who the EU recognises as the remaining state, and it's uncharted territory."
Mr Darling was speaking at an event at Barony Hall which was organised by The Herald newspaper, University of Strathclyde and the International Network of Street Papers (INSP), which supports more than 100 street papers including The Big Issue. All proceeds from the ticketed event will go to support INSP's work.