Alex Salmond was accused of lying to the public today, after an internal Scottish government document cast doubt on the feasibility of Scottish National party (SNP) plans for an oil fund.
The document, obtained under freedom of information by the Better Together campaign, shows the oil fund would require higher taxes, cuts to spending or more borrowing – contrary to public statements from Salmond.
"This is the third time in a year that the SNP have been caught out saying one thing in public while knowing the opposite was true in private," Alistair Darling said.
"First it was Europe, then it was John Swinney's leaked Cabinet paper and today it is oil.
"Last week they said they could set up an oil fund at no cost. It is now clear that their own private advice says that there would need to be tax rises, spending cuts or more borrowing – or all three.
"The SNP have quite deliberately set out to deceive the Scottish public."
Last month, Salmond said "oil is the bonus rather than the basis for the economy of an independent Scotland" but the internal report, which would have been read by the Scottish first minister, reveals that "North Sea receipts would have been required to fund public services in Scotland" since 1990.
It found that Scotland had run a net fiscal deficit in 20 of the past 21 years.
The report also raised significant doubts about Salmond's insistence that Scotland is "moving into a second oil boom".
In contrast, the report reads: "On current forecasts, it is unlikely that Scotland will run a net fiscal surplus in the years to 2016/17. There would be comparatively few opportunities to invest in an oil fund in future years."
SNP officials have insisted that the example of the Norwegian oil fund has seen the country achieve average annual returns of 5.9% over the last five years but the report found returns were volatile and that the Scottish government may not be able to cover the interest payments on its borrowing with returns on investment if it pursues the same strategy.