Parents could face higher fees and extra charges for childcare as a result of Government plans to double the free hours available to pre-school children, it has been suggested.
In addition, nurseries and childminders could put restrictions on the days and times when families can access their free entitlement, according to a poll by the Pre-school Learning Alliance.
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 Alliance's latest survey of childcare providers found that less than half of those polled (44.2%) said they plan to offer the 30-hour free entitlement, while more than a third (36.5%) were unsure and almost a fifth (19.3%) said they would not be bringing it in.
In comparison, 95.2% said they currently offer families 15 free hours for three and four-year-olds.
More than half (58.3%) of those who said they would not be providing 30 free hours said that the funding rate is not high enough, and 18.3% said it would mean reducing places.
Just over two in five (42.6%) said they are not open for 30 hours a week.
The poll asked those nurseries and childminders who said they expected the extended offer to have a negative impact on their business how they will recoup any losses from offering 30 free hours.
More than half (53.1%) said they will increase fees for any additional hours, over a third (37.2%) said they would increase fees for children of other ages and nearly half (47.9%) plan to charge for goods and services that they previously provided for free.
Two fifths (40%) said they would restrict the days and times when funded places can be accessed.
Over half (59.7%) said they were confident that they will have the capacity to meet the demand for places under the 30-hour offer, while 40.4% said they were not confident about this.
Neil Leitch, chief executive of the Pre-school Learning Alliance, said: "The promise of 30 hours of free childcare was a key part of the Conservative Party's pre-election manifesto and yet these findings show that if the Government doesn't address the sector's funding concerns, it's at serious risk of breaking that pledge.
"With so few providers currently committed to delivering the 30 hours, and so many forced to consider limiting places, raising fees or introducing extra charges in order to remain
A Department for Education spokeswoman said: "We are investing a record £6 billion per year by 2020 in childcare support and introducing a fairer early years funding formula which will see the vast majority of providers receive increased funding rates to help deliver our 30 hours free offer.
"A number of areas are already delivering our 30-hour childcare offer, with more than 5,400 places already allocated - surpassing the 5,000 target set last year.
"A further four areas are due to launch the offer this month, and we have invested £50 million capital funding to create nearly 9,000 new childcare places.
"We are also providing guidance, tools and support for providers to help childcare professionals run their businesses more efficiently."