Parents in England who claim extra free childcare from September will be asked to pay charges to help tackle a funding shortfall, nurseries have warned.
A government scheme offering 30 hours of childcare a week to working parents of three and four-year-olds is to be piloted in York next month.
But the Pre-school Learning Alliance said providers may refuse to offer the care because it could cost them money.
The government said it was set to spend a record amount on childcare.
Some families who use only free care may be unable to dictate when they want their children to attend a nursery or to secure places for more than one term at a time.
The scheme, which is also being trialled in another seven areas in September, was a key promise in the Conservative general election manifesto.
Fifteen hours of taxpayer-funded childcare for all three and four-year-olds has been available since 2010 and some two-year-olds are also eligible for free care.
But providers in York say the new policy is underfunded and they stand to lose tens of thousands of pounds.
Nurseries cannot charge for the free care promised by the government, but they can ask parents to pay towards the cost of food and special activities.
Ken McArthur, who runs a nursery in York, told BBC Radio 4’s Today programme: “I’m introducing a charge, which is something that was never there beforehand.
“I’m now going to introduce a funded hours charge, which includes the meals which we’ve been providing, in a lot of cases totally for free.”
He said parents of babies and very young children were already paying higher rates to subsidise an existing scheme offering 15 hours of childcare.
Parents who pay at his nursery can secure a place for their child until he or she reaches school age, but those who seek only free care will have to apply for a place three times a year, he said, and would not be able to dictate their hours.
Vanessa Warn, who runs two nurseries in York, said she asked parents to make contributions of £5 a day.
She said: “Our parents have been overwhelmingly supportive because again it comes down to parental choice.”
Earlier this year childcare providers threatened to pull out of the York pilot scheme until they were promised more money, but nurseries say they are still set to lose out.
Ms Warn told Today that she expected the policy to cost her £70,000 to provide next year.
The other pilot areas are Wigan, Staffordshire, Swindon, Portsmouth, Northumberland, York, Newham and Hertfordshire, but York is the only one in which the policy will be tested across the whole council.
It is scheduled to be rolled out across the rest of England in 2017.
The Pre-School Learning Alliance, a charity which specialises in early years education, warned some nurseries could opt out of the scheme.
Its chief executive, Neil Leitch, said: “As the Department for Education itself has acknowledged, if childcare providers find that they’ll be financially worse off by delivering the 30 hours, many will simply opt out of the scheme.”
But a Department for Education spokeswoman said plans to rewrite rules on how funding was distributed had received widespread support from the childcare industry.
She said: “We are doubling our free childcare offer for working parents to make it easier for them to get on and balance work with their family lives.”
The spokeswoman added that the Department for Education would be spending a record £6bn on childcare by 2020.
The Conservatives promised an extra 15 hours of free care for working parents in its general election manifesto.
Parents will be eligible for the extra 15 hours if they earn at least £115 a week.
The policy only applies in England.
In Scotland three and four-years-olds and some two-year-olds are eligible for 16 hours of care a week.