A Government scheme to give working parents 30 hours of free childcare a week could widen the education gap between rich and poor, according to a new report.
Research published by the Sutton Trust suggests the offer is being implemented at the expense of quality early years education for disadvantaged children.
Under the Government's plan, all three and four-year-olds in England will be entitled to 30 free hours of childcare a week from this month, up from 15 hours.
But the report's authors say the scheme is not adequately resourced, and a new funding formula will divert resources away from state nurseries disproportionately attended by disadvantaged children.
The research found that a third of staff working in group-based care still lack either English or Maths at GCSE, or both, at grade C or above.
The Sutton Trust criticised the Government's focus on "quantity over quality", arguing the childcare scheme could widen the gap in school readiness between disadvantaged children and their better-off peers.
The education charity's chairman Sir Peter Lampl, said: "Good quality early years provision is vital to narrow the gaps that leave too many youngsters behind by the time they start school.
"But it's unlikely that the government's policy to provide 30 hours of free childcare will provide this.
"It is understandable that the government wants to improve access to childcare for working parents. But this must not be at the expense of good early education for disadvantaged children. It is the quality of provision that matters.
"Focusing on getting it right for the poorest two and three-year-olds would make a much bigger difference to social mobility, by improving their chances at school and in later life."
The report, entitled Closing Gaps Early, by Dr Kitty Stewart of the London School of Economics (LSE) and Professor Jane Waldfogel of New York's Columbia University, looks at early years policy in England.
It states that progress made in closing the gap in school readiness between disadvantaged and advantaged children could be put at risk by the Government's approach, calling on ministers to reverse the policy unless the funding is in place to ensure quality is maintained.
Dr Stewart said: "Recent changes to funding for early education look set to have further damaging effects on the quality of provision. The new 30-hour offer is not adequately resourced, meaning money will be spread more thinly.
"To make up some of the funding gap, a new funding formula reallocates resources away from state nurseries disproportionately attended by disadvantaged children, and they may in the future struggle to afford a qualified teacher.
"To remove this advantage must be expected to have negative effects on social mobility."
Prof Waldfogel added: "We are also concerned about the proposed cuts to family benefits - since we know that economic security is critical for children, especially in the early years."
A Department for Education spokesman said: "Every child should receive the same high-quality care and support, regardless of their background or where they live, which is why we are increasing spending on childcare to over £6 billion per year by 2019-20 .
"Alongside our 30 hours free childcare offer, we are spending over £2.5bn on free childcare for disadvantaged 2-year-olds over five years, as well as providing extra support for disadvantaged families through the Early Years Pupil Premium.
"Our independent evaluation of the early delivery areas showed that 30 hours did not have any significant adverse effects on the 2 year old offer. This evaluation also showed the 30 hours offer supported families by taking huge pressure off their finances and helping them increase their working hours. "
Kevin Courtney, joint general secretary of the National Education Union, said: "England has a tradition of high-quality early years education. This report is the latest confirmation that government policies are destroying it.
"As this report points out, high-quality early years education that is properly funded and run by a qualified teacher is essential to ensure every child has the best start in life.
"This aim however is being undermined by government policy, with funding in the early years being cut in real terms and the requirement for a qualified teacher in every school nursery and reception class removed."
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