The public spending watchdog has questioned plans to fund increased nursery provision in Scotland.
Audit Scotland said in a report there was a "significant risk" councils would not be able to resource a planned expansion.
The Scottish government has promised to almost double the annual hours in paid-for nursery places to 1,140.
In response to the report, the government repeated its pledge to fully fund the policy.
Ministers plan to meet the new target for nursery provision by August 2020.
It is the equivalent of about 30 hours per week during term time.
The watchdog's report, which was prepared for the Accounts Commission and the Auditor General, said: "There are significant risks that councils will not be able to expand funded early learning and childcare (ELC) to 1,140 hours by 2020.
"In particular it will be difficult to increase the infrastructure and workforce to the levels required, in the limited time available.
"The Scottish government should have started detailed planning with councils earlier, given the scale of the changes required."
The report drew attention to a gap between council and government estimates of the cost of the policy.
Councils expect spending on the policy to hit £1bn by 2021-22. The indicative government figure is £840m.
Local authorities have estimated they will need 12,000 whole-time equivalent (WTE) staff, while the government believes between 6,000 and 8,000 WTE staff will be needed by 2020.
The report also examined the 2014 expansion of funded places to 600 hours.
It found the government has invested almost £650m but did not plan how to evaluate the outcomes, meaning "it is not yet clear whether the investment is delivering value for money".
Accounts Commission chairman Graham Sharp said: "The scale of change needed over the next two years is considerable and there are significant risks that councils will be unable to deliver that change in the time available.
"There is now an urgent need for plans addressing increases in the childcare workforce and changes to premises to be finalised and put in place."
'Not clear enough'
Caroline Gardner, Auditor General for Scotland, added: "Focusing on the early years has the potential to make a real difference to young people's lives but the Scottish government was not clear enough about what the expansion of funded hours in 2014 was expected to achieve.
"We are encouraged that the Scottish government is now planning better for how it will assess the impact of the expansion to 1,140 hours and has already published some baseline information."
Minister for Childcare and Early Years Maree Todd said the government remained "on track to deliver 1,140 hours by 2020".
She said: "It is not unusual, at this point in the life of a major project, for people to have different ideas as to the final cost. What is not in doubt is that the Scottish government has pledged to fully fund this policy.
"That's why we are working with councils to help them develop their expansion plans and have recently reached agreement with Cosla on the process of arriving at the multi-year funding needed.
"While this work is under way, we have already increased capacity in early years courses in colleges and universities and are investing in the significant expansion of the workforce needed to deliver the expansion."
The local authority body Cosla has welcomed the Audit Scotland report.
Spokesman Stephen McCabe said: "Scotland's councils are dedicated to getting it right for children and young people and no more so than when we are delivering services for our youngest children and their families.
"We know that it is at this point that we can make the greatest difference to the outcomes in their lives.
"Cosla are pleased that the quality of local authority provision is recognised in this report as is the continual growing of flexible ELC provision within each council area."
Conservative education spokeswoman Liz Smith said the report was "damning in terms of exposing the failures of SNP policy on child care".
For Labour, education spokesman Iain Gray added: "This independent report blows a £160m black hole in a flagship SNP government policy.
"Raising the amount of free childcare families are entitled too has been a cornerstone of the SNP's offer to families - but this report reveals that, not only does the Scottish government not expect to fully fund it, but that it is also miles behind the levels of staff recruitment needed."