Parents could face higher fees and extra charges when the government rolls out a plan to double the number of free childcare hours, warn providers.
From September, three and four-year-olds in England will be entitled to 30 free hours of care a week in term time.
But a Pre-school Learning Alliance poll of childcare providers suggests many will struggle to offer the 30 hours as the funding will not cover their costs.
Ministers say affordable childcare is at the heart of their agenda.
Of 1,332 childcare providers questioned last month, only 44.2% said they definitely planned to deliver the 30 hours, compared with 95.2% who said they were providing the 15 hours currently funded by government, while more than a third (36.5%) were unsure whether to offer it.
Almost a fifth (19.3%) said they would not be introducing it and more than half of those (58%) said it was because the funding was not high enough.
About two-fifths of nurseries and childminders said they would have to put restrictions on the days and times when families can take their free hours.
Of nursery owners expecting to make a loss under the scheme, more than half said they would have to increase fees for any additional hours.
More than a third (37.2%) said they would increase fees for children of other ages and nearly half (47.9%) would charge for goods and services that they previously provided for free.
More than half (59.7%) said they were confident that they would have the capacity to meet the demand for places under the 30-hour offer, while 40.4% said they were not.
Neil Leitch, chief executive of the PSLA, 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 sustainable if they do offer it, many parents expecting easy access to a ‘free’ 30 hours place in September are likely to be disappointed.
“The fact is that the 30 hours offer cannot succeed without adequate investment. Neither parents nor providers should be expected to pay for a promise that government chose to make.”
A Department for Education spokeswoman said that by 2020 the government would be investing “a record £6bn per year… 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”.
The spokeswoman added that pilots of the scheme were already successfully under way in a number of areas, with more due to launch this month.
“We are also providing guidance, tools and support for providers to help childcare professionals run their businesses more efficiently,” she said.