Grammar schools are four times more likely to admit private school children than those on free school meals, according to a report by the Institute for Fiscal Studies.
The report, sponsored by the Sutton Trust, found that in selective local authorities, 3% of grammar school entrants were eligible for free school meals. At other state schools, the figure is 17.5%, the Guardian reports.
At the other end of the spectrum, grammar schools were admitting 12.7% of their children from outside the state sector, largely from independent schools, the report said. On average, 6% of primary school children are enrolled in a private school nationally.
The findings, which form part of a larger report, Poor Grammar: Entry into Grammar Schools for disadvantaged pupils in England, indicate that even after allowing for "a wider range of factors that may depress pupils' academic achievement", sizeable differences remain between entrance rates.
The report added that on other measures of deprivation, the differences in admission "extended up the income scale".
"Four per cent of pupils in grammar schools live in the poorest fifth of neighbourhoods, around 21% come from the middle quintile and 34% live in the richest fifth of neighbourhoods," it said.
The chairman of the National Grammar Schools Association, Robert McCartney QC, dismissed the report, saying he did "not see anything significant in these conclusions".
He said there was "phenomenal competition" for the handful of grammar school places and after producing some of the best GCSE and A-level results in the UK, they had become a victim of their own success, which could explain discrepancies in admittance rates.
"You get a preponderance of aspirational parents [applying], particularly from the middle classes," he said, adding that these families often apply from outside areas where there were no grammar schools, further skewing figures.
He also criticised the free school meals measure. "As a yardstick for social deprivation, it is generally recognised as a crude instrument ... A number of families, particularly aspirational parents who are not well off, do not apply for free school meals.
"The type of parent who applies to grammar school, whether working class, middle-class whatever, are aspirational. They are parents who ... are determined to get the best education possible as they see it, for their children," he said.
"Many, many parents from deprived areas, including what is generally called the dependency classes, are essentially not particularly interested in any form of academic education. Their interests are directed towards pop culture, sports."
The report's author, Anna Vignoles, said the numbers coming into grammar schools from outside the state sector were "extremely high".
She said: "Even if you took two identical kids, with similar levels of achievement, and they're both coming from a state school, it's still the case that the child on free school meals is significantly less likely to get into the grammar school."