Many nurseries and childminders are struggling with the cost of offering families 30 hours of free childcare a week, a poll suggests.
Under a Government scheme rolled out last autumn, three and four-year-olds in England with working parents are entitled to this amount of free care.
But a new small-scale survey, conducted by the National Association of Head Teachers (NAHT), suggests that while the majority of early years providers are offering the scheme, a lack of funding means that many are having to find extra funds to cover the costs from other sources.
Overall, more than three in four (77%) of the 425 people polled said that they are delivering the 30 free hours offer.
Of these, 76% said that they only offered the entitlement during school hours - typically around 9am to 3pm.
Just under a fifth of those questioned (19%) said that the funding they receive for the free hours is enough to cover their costs, with nearly three in five (59%) saying it is insufficient.
Asked how they were balancing their budgets if they were not getting enough money, 70% said that they were using funds from another part of their school or setting, while 16% said they were charging parents for "extras" related to childcare and 18% said they were charging mums and dads a higher rate for additional hours.
Paul Whiteman, general secretary of school leaders' union NAHT, said: "Most of the respondents to our survey are finding that they can only make things work by borrowing money from other parts of their budget. This is unsustainable. Budgets are at breaking point. Increased costs are hitting all education settings, from nurseries to primary schools, to secondaries, special schools, and colleges.
"The Government's 30 hours free childcare promise should be a boost to parents. But it is not truly free. We are seeing that providers are having to make up the funding shortfall by charging parents a higher rate for additional hours outside the school day, or for extras, like milk or nappies.
"Parents only using their free hours are confused to find they are still being asked to make 'contributions'. And parents who need wraparound provision will not find their costs dropping by anything like 30 hours. While all parents will see some benefit, enough childcare to meet a full working day is still prohibitively expensive for many."
Councillor Roy Perry, vice-chairman of the Local Government Association's children and young people board, said: "Since last September, councils and providers have been delivering the Government's commitment to an additional 15 hours of free childcare for working parents, bringing the total to 30 hours.
"However this report reaffirms concerns councils have had that the funding behind the scheme is not enough to secure this provision for everyone who wants it.
"We have previously warned about the impact on quality, with a risk that insufficient funding will lead providers to employ less qualified staff or struggle to provide enough support for children with additional needs or disabilities."
A Department for Education spokesman said: "We know the cost of childcare can be a burden on families, which is why we are spending more than any other government on support - around £6 billion a year by 2020 - to help make sure that every child gets the best start in life.
"Couples need to earn just over £13,000 a year and lone parents £6,515, to be eligible for 30 hours of free childcare. Those who aren't eligible can access a range of other benefits - for example parents who are working a few hours a week may be eligible to claim back 85% of their childcare costs through Universal Credit."
The poll questioned 425 people, mainly leaders in primary and infant schools with nursery classes as well as nurseries and childminders in February and March.
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