A report into the state of childcare by early years research company Ceeda, shows that childcare places on non-domestic premises were available to just a quarter of children living in the 20 per cent most deprived local authority areas, compared with 33 per cent in 2016.
Conversely, among the least deprived areas availability has increased from 32 per cent in 2016 to 43 per cent now.
The report suggests the decline is due to nearly a decade of cuts to local children's centre provision.
Since 2009, around 1,000 children's centres have closed according to research published in April by education charity the Sutton Trust.
The report also shows a widening gap between childcare availability and quality in the wealthiest and most deprived areas.
Ofsted rated only 19 per cent of non-domestic settings in the fifth most deprived areas as "outstanding" by March 2018, compared with 25 per cent in the least deprived.
Similarly, 10 per cent of childminders in the most disadvantaged areas were rated outstanding, half the rate of the richest areas.
The research also highlighted the performance gap between the most and least attaining children - which had been reducing year-on-year from 2013 - levelled off in 2017.
In terms of funding, the 40 per cent most disadvantaged two-year-olds, who are eligible for government-subsidised childcare, are shown to have had their funding cut in real terms.
The report suggests the rate provided by the government is £0.44 less than it would have been had it followed inflation, indicating a real-terms cut of 8.5 per cent.
This means providers receive almost a third less money than is needed to deliver places for disadvantaged two-year-olds.
Pre-school Learning Alliance chief executive Neil Leitch called on the government to invest more to address the imbalance in provision.
"How can the government talk about the importance of social mobility when chronic childcare underfunding has meant that those families with the greatest need, and those children who would benefit most from high-quality provision, are the ones who face the most difficulties accessing places?" he said.
"If ministers are genuinely serious about ‘closing the gap' and improving the life chances of the most vulnerable children, it simply must invest what's needed to support a quality, accessible and sustainable childcare system in this country."
His call was backed by Ceeda managing director Jo Verrill, who also warned that conditions in the childcare sector were set to worsen.
"Declining sector capacity in areas of disadvantage is a major cause for concern, and is set to get worse as provider costs increase and funding rates stand still," she said.
"Providers operating in deprived areas are least able to off-set losses on funded hours with private fee income, and it is children who stand to gain most from high-quality early education that are at risk of losing out."
Purnima Tanuku, chief executive of National Day Nurseries Association (NDNA), said: "We have seen more smaller settings close since the 30 hours policy was brought in, an increase of 66 per cent compared with the previous year.
"More children than ever before are living in poverty and these are the children who need high-quality care to improve their life chances. If the government is serious about social mobility, these issues must be addressed. Government needs to fully support the early years sector, its staff and the families they serve particularly in poorer communities."
A Department for Education spokeswoman said: "We want every child to have the best start in life, with access to high-quality childcare and early years education.
"More than nine in 10 childcare settings across the country are rated good or outstanding by Ofsted, and the proportion of high-quality provision available in the least and most deprived areas is similar.
"Formal childcare or early education can have a positive impact on a child's early development, which is why we are investing £30m to expand the number nursery places run by good schools in disadvantaged areas, so more children can access a high-quality early education."