Tens of thousands of young children attending nurseries could miss out on 30 free hours a week of childcare, industry leaders have said.
More than a fifth of nurseries (22%) say they will not be, or are unlikely to take part in the Government scheme, which will see free childcare hours for three and four-year-olds in England double later this year, according to a new survey by the National Day Nurseries Association (NDNA).
It calculates that this means that around 51,000 eligible children will not be able to access a nursery place for the full 30 free hours.
This is based on a Government estimate that around 390,000 youngsters will qualify for the offer, and that 60% of childcare places are provided by private, voluntary and independent nurseries. Other places are offered by providers such as playschemes.
The NDNA's latest survey found that 44% of the daycare providers questioned said that they will offer families the free 30 hours, while around 35% are unsure.
Under the Government's plans, from this autumn, all three and four-year-olds in England will be entitled to 30 free hours of childcare a week, up from the current 15 hours.
But industry leaders have raised concerns that nurseries and childminders will not get enough public funding to cover the costs of providing these places.
The NDNA survey, which only questioned day nurseries, found that 83% plan to increase their fees this year, with an average hike of 4.5%, while 71% reported raising their fees in 2016, with an average rise of 4%.
The association's chief executive Purnima Tanuku said: "Our findings are significant as they concern full daycare settings, the largest part of the nursery sector, who provide more places than schools or playgroups.
"There is no such thing as 'free' childcare - parents, nurseries and their staff are all paying for this.
"It is time the Government stopped promising parents 'free' childcare hours unless they are prepared to invest the money needed. This manifesto promise is in real danger of failure."
The Government announced last week that is making an extra £50 million available to help nurseries, pre-schools and playgroups to upgrade and build new facilities. The money will create more than 9,000 additional childcare places, it said.
:: NDNA questioned 788 nurseries in England in January and February.
A Department for Education spokeswoman said the Government's 30 hours childcare offer is already working in towns across the country.
"We've seen great success with our Early Implementers, which surpassed the target of 5,000 places set last year and four more areas will launch the offer later this month.
"Helping families access affordable childcare is at the heart of this government's agenda.
"Following our own survey, most said they would offer 30 hours, even before we introduced the Early Years National Funding Formula which will increase the funding rates for providers in the vast majority of local authorities, part of our record investment of £6 billion per year into the sector."