A survey of all 152 English councils by the Family and Childcare Trust on what impact they expect from the change to free childcare entitlements found that 44 per cent of councils believed that the changes will result in reduced financial sustainability for some settings.
This compares with just nine per cent that believe it will not have a negative impact, with 47 per cent saying they are not sure what impact it will have.
The study also highlighted concerns about whether families eligible for the 30 hours will even be able to access it. Only 33 per cent of local authorities expect there to be enough childcare available for eligible three- and four-year-olds, with just over half (54 per cent) not yet sure whether or not there would be enough.
Meanwhile, 44 per cent of local authorities saying that the 30-hour offer would reduce the financial sustainability of some settings.
Separate research published today by Fact - No Shortcuts: Quality and the Free Childcare Extension - found that childcare providers and local authorities are worried that the policy change could mean that there was less childcare available, reduce the quality of care, and that settings might find it harder to meet the needs of children with special educational needs and disabilities.
Ellen Broomé, deputy chief executive of the Family and Childcare Trust said: "We know that only high-quality childcare helps boost poorer children's learning. And while working parents are pleased to receive more hours of free childcare, they are not willing to cut corners on childcare quality.
"As this policy rolls out, the government must make sure that all families are able to access the high-quality, affordable childcare that they need."
Under government plans, working parents of three- and four-year-olds will be entitled to 30 hours per week of free childcare for 38 weeks per year, an increase on the current 15 hours.
Richard Watts, chair of the Local Government Association's children and young people board, said: "Councils are committed to ensuring that parents have access to high-quality, flexible childcare so that all children get the best start in life.
"From September, councils and providers will be delivering the Government's commitment to an additional 15 hours of free childcare for working parents, bringing the total to 30 hours.
"However, councils remain concerned that the proposed increase in funding will not be enough to secure this provision for everyone who wants it.
"Government must use the learning from local areas that have been piloting the offer to ensure that there is no reduction in the quality of early education and care provided.
Neil Leitch, chief executive of the Pre-school Learning Alliance, said: "The findings of the Family and Childcare Trust's report, while not surprising, are deeply concerning."
"It is simply not good enough that, as a result of the government's continued refusal to listen to funding concerns, the vast majority of local authorities cannot say with any certainty whether or not they will have enough places for eligible three- and four-year-olds with only seven months to go until the 30-hour offer is implemented.
"We have long warned that unless the scheme is properly funded, providers are likely to either limit the funded places they offer, or pull out of the scheme altogether. Add to this the fact that recent research by Ceeda commissioned by the Alliance showed the government has significantly under-estimated the level of demand for 30-hour places, and it's clear that we are heading towards a capacity crisis.
"If the government wants to make good on this manifesto pledge, then they urgently need to invest what's needed. Time is running out, and at this rate, come September, we are going to see a lot of disappointed parents who are unable to find a 'free' place for their child as promised."
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 £6bn 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 five thousand parents are benefitting from and just recently we announced £50m 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."