Childcare proposals for an independent Scotland "don't add up", according to analysts at the Treasury.
The UK Government department has challenged one of the central policies of the Scottish Government's independence plan as part of its analysis of the fiscal implications of a Yes vote in the referendum.
The Scottish Government has pledged to provide all three and four-year-olds, and vulnerable two-year-olds, with 1,140 hours of childcare per year by 2020.
It estimates that the policy could result in about 104,000 women entering employment and an additional £700 million in tax revenue which would help pay for the additional childcare.
The Treasury's analysis says the Scottish Government's figures are flawed.
Office for National Statistics (ONS) data suggests that about 39% of mothers with children aged up to three are not working.
The Scottish Government estimates that 212,000 mothers could be affected by the policy, with ONS calculations suggesting around 83,000 of this population are not employed.
Treasury analysts said: "The Scottish Government's numbers simply don't add up.
"Even if every mother out of work moved into work - in itself highly unlikely - there would still be a shortfall of 21,000.
"Their costing is worse than wishful thinking, the sums don't work."
The Treasury's analysis comes after a paper by the Scottish Parliament Information Centre (Spice) published in April found that the Scottish Government had provided no evidence that its plan would get more women out to work.
A spokesman for Scotland's Finance Secretary John Swinney said: "In terms of our childcare commitments and boosting the number of women in work, it is our commitment to stop wasting money on Trident and on contributing to the running costs of Westminster that gives us the ability to invest in these other priorities."
Chief Secretary to the Treasury Danny Alexander said: "This is typical of the nationalist elite who think they can con voters by inventing mothers that don't exist and then assume that they will all get back into work.
"They will say anything, no matter how far-fetched, to get their way."