The Government's plans to expand the academies programme risk increasing the numbers of poor pupils missing out on the best schools, a report by Barnardo's has warned.
Schools that are handed control over their own admissions, such as academies and free schools, are more likely to be socially selective than state maintained schools, the children's charity said.
The study, published today, states that children born into poorer families are "condemned to the worst schools" with unfair admissions practices leading to "skewed" intakes that do not reflect the population of the surrounding area.
The charity is calling for the implementation of a nationwide "banding" system, which would mean schools admitting equal amounts of children of different abilities. It also wants the Government to require Ofsted to look at all schools' pupil intake.
Martin Narey, Barnardo's chief executive, said: "Secondary school admissions fail to ensure a level playing field for all children. Instead we are seeing impenetrable clusters of privilege forming around the most popular schools.
"Allowing such practice to persist - and almost certainly expand as increasing numbers of schools take control of their own admissions - will only sustain the achievement gap in education and undermine the prospects of the most disadvantaged and vulnerable children.
"If we are to wipe out the entrenched poverty that erodes the life chances of one in four children in the UK, we must stop educational disadvantage being passed down from one generation to the next."
The Barnardo's report states that just 15 per cent of schools had control over their own admissions in 1988, but by 2009 that figure had almost tripled to 42 per cent. This is expected to rise substantially under the Government's plans to expand its academies and free schools policy.
Mr Narey said the current admissions process is too complex for many parents, and called on the Government to establish clearer accountability for schools that become their own admissions authorities. He also recommends separating the responsibility for setting school admissions policy and administering it.
Professor Anne West, an expert on secondary school admissions at the London School of Economics, said it was too early to say what kind of impact the new academies and free schools would have.
"It really depends on how the academies and free schools shape up," she said. "I do think there is legitimate cause for concern. You may have sponsors who want to open up local community schools. But you might have a school that has a particular ethos, which parents of children could find particularly tricky."
A spokesman for the Department for Education said the Government is taking action to improve opportunities for pupils from poorer homes.
"We are introducing a pupil premium, reforming the admissions system to make it simpler, and getting the best teachers in disadvantaged areas by increasing investment in Teach First," he said. "Academies are socially comprehensive schools that are doing very well and many have embraced socially comprehensive admissions criteria such as banding."