More than half of councils are unsure if they will have enough childcare available when plans to double the free hours available to pre-schoolers comes into force, a survey suggests.
Under major reforms from this September, three and four-year-olds in England will be entitled to 30 free hours of care a week during term time - twice as much as they currently receive.
But a new poll of local authorities in England, conducted by the Family and Childcare trust, found uncertainty about the impact of the change.
Around 54% of those questioned said they did not know if they would have enough childcare available for youngsters using the 30 hours, while a further 13% there would not be enough.
A third (33%) said that there would be sufficient places.
More than two thirds (67%) said they thought some childcare providers in their area will not offer the entitlement to 30 free hours of care, while more than half (56%) thought that the change would mean more families were able to work in ways that meet their needs
The poll also asked if councils believed the reform would lead to a dip in quality in early education, with two thirds (66%) saying it would make no difference, 32% unsure and 2% suggesting quality would be affected.
There were concerns about the future of childminders and nurseries, with 44% of the councils polled saying there could be "reduced financial sustainability" of childcare providers.
And there were questions over whether the move would lead to increased costs, outside of the free hours available.
More than half (56%) did not know if there would be increased costs for three and four-year-olds, while 63% were unsure if it would lead to higher costs for children aged two and under.
More than a third (37%) said it would increase costs for three and four-year-olds and 23% said it would do so for younger children.
The report says: "There was a lack of certainty about the availability of the 30 hour offer for eligible families.
"Only a third of local authorities expect there to be enough childcare available for three and four year olds, with just over half not yet knowing whether or not there would be enough.
"A clear majority of local authorities expect some settings not to offer the 30 hour entitlement - presumably because it would not make financial sense for them to do so.
"Until the policy has been introduced in full, it is difficult to predict how many settings this will apply to: it is likely that the pattern will vary in different parts of the country based on the comparative price paid for funded and non-funded places, and settings' ability to attract parents without offering the free entitlement.
"Despite this uncertainty, the majority of respondents believe that the 30 hour entitlement will enable more families to access childcare which meets their needs."
A Department for Education spokeswoman said: "Helping families with high quality, affordable childcare is at the heart of this Government's agenda.
"That's why we are investing a record £6 billion per year by 2020 and our new fairer funding formula will mean the vast majority of providers receive increased funding rates.
"A number of areas are already delivering our 30-hour childcare offer, which almost 5,000 parents are benefitting from, and just recently we announced £50 million to create nearly 9,000 new places.
"When the scheme is rolled out nationally, around 390,000 working families will be eligible and, in many cases, these children are already in existing childcare places, which they will simply get for free when our offer goes live from September."