A major recruitment drive is needed to ensure an expansion of funded nursery places is to hit its summer deadline, a spending watchdog has warned.
The national entitlement to free childcare will increase from 600 to 1,140 hours in August.
Audit Scotland said more than 4,000 nursery staff would need to be recruited and half the building work was not yet complete.
The Scottish government said it was confident the deadline would be met.
The expansion of free childcare is part of a "landmark" funding deal between the Scottish government and local authorities.
Councils, which have been given £567m to deliver the nursery pledge, have recruited the equivalent of 4,310 full-time staff - approximately half the number required.
In a review of progress, Audit Scotland said the plans were on track - but a great deal of work would need to be done over the summer months.
Caroline Gardner, Auditor General for Scotland, said: "The Scottish government and councils have worked well together to increase early learning and childcare hours, and we've also seen improvements in how the project will be evaluated.
"But the timeline remains tight and there are big risks around infrastructure and workforce."
A phasing system was implemented last year giving approximately 50,000 children 1,140 hours of funded childcare ahead of the national rollout.
The Scottish government said by the end of the expansion, more than 900 nurseries will have been built, extended or refurbished since 2018.
Children's Minister Maree Todd said: "We are committed to delivering the roll-out from August and councils have contingency plans in place for all critical projects due to complete this summer, so we can be confident that the places will be there to deliver the expansion for Scotland's children.
"The expansion also opens up more opportunities for playgroups, private and third sector nurseries, and childminders to be involved in delivering funded hours."
The Scottish Conservative education spokesman Jamie Green said that necessary work had been "left to the last minute".
He added: "The first minister promised the world to thousands of parents across Scotland when it came to free childcare. They will deliver a very damning verdict on her SNP government if this promise isn't up-and-running on time.
"And what ministers seem to have forgotten through all of this is that good childcare isn't just about the number of hours on offer, it's the flexibility within that."
In 2017 the Scottish government announced it would almost double its funded childcare as part of a plan to reduce health, education and employment inequalities later in life.
Audit Scotland later warned of a "significant risk" that local authorities would not be able to fund the expansion.
However, the government and Cosla reached an agreement two months later. It set out plans to spend £990m on day-to-day funding for the scheme by 2021 - £150m more than the government's previous estimate.
Opposition politicians had previously warned of a £160m "black hole" in the funding proposals.
Funding will rise annually from £33m this financial year to £567m by 2021/22, totalling £1.5bn over five years.
The National Day Nurseries Association (NDNA) produced research last year which found that 71% of employers struggled to recruit staff at practitioner level - and that Scotland's nursery staff turnover rate was 29%, higher than the UK average.
It recommended the scheme be reviewed annually to ensure nurseries could cover the cost of the new offer as well as keeping staff.