England's top secondary schools are fast becoming the preserve of the middle classes who can afford to pay a premium to live in the right area, it has been suggested.
New research shows that poorer youngsters are much less likely to win places at the 500 comprehensives which get the best GCSE grades, including English and maths.
These secondaries are significantly more "socially selective" that the average state school, the Sutton Trust study concludes.
It warns that these schools admit around 9.4% of pupils eligible for Free School Meals (FSM) - a key measure of poverty, compared to 17.2% attending the average state school.
"There is a marked difference in the proportion of pupils from poorer backgrounds attending the top 500 comprehensives compared to the national average when based on the 5A*-C English and maths measure, with FSM rates just over half of the national average in comprehensive schools," the report says.
Around half of this gap is down to schools having catchment areas that have lower numbers of disadvantaged pupils, but the rest is due to social selection, researchers argue.
More than eight in 10 (85%) of the top 500 schools take fewer poorer pupils than they should do, given the numbers living in their catchment area.
The study also finds that there is a house price premium of around 20% attached to living in the right area for a top comprehensive school. A typical house in one of these catchment areas costs around £45,700 more than the average property in the same local authority.
This means that pupils whose families can afford to buy in these coveted areas are more likely to get a place at one of the top secondary schools, effectively pricing poorer youngsters out.
Faith schools were the most socially selective group of top schools, the study concludes, making up 33.4% of the top 500, based on A*-C grades, including English and maths.
The findings, which come on the day that children across the country are told which secondary school they have been allocated for this autumn, did show that the situation has improved slightly - in 2013, the average proportion of poorer pupils in the best schools was 7.6%.
Researchers also looked at the Government's new Progress 8 measure - which looks at the progress and achievement of a pupil over eight subjects and found that the schools with the best results on this measure had FSM rates of 15.2%, closer to that of the average state school. This makes them less socially selective, the study says, with around a third of these schools admitting more FSM pupils, relative to their catchment area.
Sutton trust chairman Sir Peter Lampl, said: "Getting a place at a high attaining school is key to getting on in life. Yet the bottom line is your chances of doing that depends on your parents' income and whether they can afford the extra £45,700 house premium to live in the catchment area.
"This is why we want to see more use of ballots - where a proportion of places is allocated randomly. Ballots would ensure that a wider mix of pupils would get into the best schools."
A Department for Education spokesman said: "Selection by house price is simply unfair, which is why we've already set in motion plans to tackle it.
"We plan to create more good school places in more parts of the country by scrapping the ban on new grammar schools, as well as harnessing the expertise and resources of our universities, and our independent and faith schools.
"This will build on the work of the last six years that has seen the number of children being taught in schools that are rated good or outstanding rise by almost 1.8 million - but we want to go further.
"We have announced 12 Opportunity Areas across England, backed with £72 million investment, where we are working to break down the barriers to social mobility that too many still face."