The Scottish government's promise to provide free childcare in an independent Scotland, which was one of the pillars of its recent White Paper, will cost £1.64bn a year by the early 2020s.
According to analysis by the British Treasury, the SNP's blueprint for an independent Scotland does not balance financially.
"This detailed Treasury analysis shows the Scottish government would need to find £1.6bn a year in higher taxes or spending cuts to fund just some of the additional commitments set out in the White Paper. The reality is that the White Paper shows nothing about how they would pay for these commitments," said Danny Alexander, chief secretary to the Treasury.
The SNP's "transformational childcare" proposal contained in the paper promised to create 35,000 jobs in the childcare industry, while bringing in about £700m ($1.1bn, €835m) per year in tax revenues.
Other SNP initiatives included cutting corporation tax by 3% and reducing air passenger duty by 50%.
The Treasury claims that these will cost £1bn a year by the end of the first parliament and £1.6bn by the end of the second parliament.
However, the deputy first minister Nicola Sturgeon said Alexander's claims amounted to scare-mongering.
"Danny Alexander's figures are all over the place. In September, the No campaign were claiming a funding gap of £32bn, by last month that had shrunk to £10bn, and now they are saying it is £1.6bn – all these figures are wrong, but at this rate even the No campaign will be predicting a healthy surplus by the time the referendum arrives!
"This analysis completely ignores the dynamic impact of the policies we are proposing to increase economic activity, which will boost growth and tax revenues.
"But the most serious point behind all this is that the vastly different choices in spending priorities has now been laid bare. Westminster is pressing ahead with a new generation of weapons of mass destruction, at a cost of up to £100 billion, and at the same time is attacking the Scottish Government's plans to transform childcare in an independent Scotland," she said.