An analysis by the politically neutral Scottish Parliament Information Centre challenged the Scottish Government's claim that providing free nursery care for all one to five-year-olds would draw an extra 104,000 women into the labour market.
The study said there were only 64,000 mothers of under-fives in Scotland who were unable to work as a result of looking after their children. It added that, of those, just 14,000 indicated they would like to start work.
The figures - which showed a further 129,000 mothers of under-fives were working while 15,000 were unemployed but looking for a job - raised questions about the sustainability of the childcare plans.
Ministers believe that providing full-time childcare for all children aged from one until they begin school will increase female participation in the labour market to the same level as Sweden, equivalent to a rise of 6% or an extra 104,000 women in work.
If achieved, the increase would generate an extra £700million in taxes that would help pay for the childcare.
The SNP have never put a price on the policy but yesterday's Information Centre report calculated a total cost at £1.2billion, in line with previous unofficial estimates.
The report also said there was no direct evidence that providing free childcare would increase female employment to the same level as Sweden.
It said: "The Scottish Government analysis does not consider whether the proposed changes in childcare policy would lead to such a change, rather it seeks to estimate what might happen if such a change in female participation took place.
"The increased output and tax revenues suggested by this analysis rely on up to 104,000 women moving from economic inactivity into economic activity.
"At present, there are only 64,000 mothers of one to five-year-olds who are economically inactive."
Labour MSP Neil Bibby said: "This analysis by the Scottish Parliament's independent experts blows apart Alex Salmond's flagship childcare proposal.
"No full costings, no economic analysis, no financial modelling and the fact they are at least 40,000 mothers short shows just how amateur and ridiculous the Scottish Government's policy making has become. The SNP has misled Scottish parents. Scotland deserves better than this."
The childcare pledge is at the heart of the SNP's economic vision for an independent Scotland, set out in its White Paper, Scotland's Future. The blueprint promises to phase in the policy by 2024.
A Scottish Government spokesman said: "The proposal to transform childcare will benefit women in Scotland year on year, including many who may not currently have children or who are currently in work - something highlighted in our own report.
"Getting more parents into work will mean more tax revenues can be collected and put back into the economy and we would be doing this by removing what parents have told us is the biggest barrier - poor access to affordable, flexible childcare.
"Independence is the only way to secure a transformational universal system offering childcare from the age of one that would respond to the aspirations of parents, employers and society as a whole."