The 30-hour free childcare scheme could face a shortage of tens of thousands of spaces, new figures suggest.
Early years organisation Pre-school Learning Alliance has warned that findings from an independent report indicate the Government has underestimated the number of families likely to be eligible for the scheme.
The Government has estimated that 390,000 three and four-year-olds will qualify for the 30-hour offer, which is restricted to children from working families meeting certain earnings thresholds.
Research based on a survey of 1,708 households suggests that the number of children currently meeting the eligibility criteria is 478,000 - 23% higher than the Government's estimate.
It was commissioned by the organisation and carried out by Ceeda ahead of the scheme's September launch.
The findings further suggest current estimates do not take into account the impact of parents in working households making relatively small changes to their work patterns to become eligible for the scheme.
According to Ceeda, such moves could account for a further 22,000 children becoming eligible for the offer, bringing total eligibility to around 500,000 overall - 28% higher than the Government's estimate.
Neil Leitch, chief executive of the Pre-school Learning Alliance, said: "These figures are deeply concerning and suggest that the Government has significantly underestimated the likely demand for places under the 30-hour offer."
He added: "The Government must do more to support early years providers if the 30-hour scheme is to have any chance of working in the long-term.
"That means both adequately funding the creation of enough new places, and ensuring that the free entitlement offer in general is funded sufficiently in the long term.
"Thirty hours of so-called 'free childcare' may sound like a great policy, but if there aren't enough places to match demand, and the Government continues to refuse to listen to valid concerns over funding, the policy simply cannot succeed."
In its research Ceeda did not include parents from workless households moving out of unemployment and into work in its top-line figures on the demand for childcare places.
This was because of the statistical uncertainty around whether intentions to return to work would necessarily translate into actions, and the additional challenges to taking such a step, such as a lack of available jobs.
Under Government plans, free childcare and early-years education for three and four-year-olds in England is to be doubled from 15 to 30 hours for each week of the school year.
A Department for Education spokeswoman said: "We are investing more in childcare than any previous government, spending a record £6 billion per year by 2020 - this includes an additional £1 billion per year on our free entitlements to support families with the cost of childcare.
"Around 390,000 working families will be eligible for 30 hours free childcare from September.
"In most cases, their children are already in existing childcare places, which will become free. We are also creating almost 9,000 new places through £50 million of capital grants."