Grammar schools were meant for the brightest irrespective of background
Grammar schools take relatively fewer bright, poor pupils than schools that do not select by ability, a study says.
Research for the Sutton Trust suggests other state schools in England take twice the rate of smart, poor pupils compared to grammar schools.
The report also suggests that state faith schools take fewer poorer pupils compared with other state secondaries.
The government said the mandatory admissions code introduced last year outlawed unfair admission practices.
The researchers based at Durham University analysed the pupil characteristics of every child who began secondary school in England in 2001.
They found that about 2% of grammar school pupils were from low income families - on free school meals - compared with 12% of pupils at non-grammar schools.
This is largely because those attending grammar schools have to pass an academic test and only one in 20 pupils among the top performing pupils are on free school meals, they said.
However, in grammar schools only 2% of the top achievers were on free school meals, compared with 5.5% for non-grammar schools.
They also compared the rate of pupils on free school meals within schools with that of the areas from which those pupils came.
And a large number of non-academically selective state schools were found to be more socially selective than grammar schools.
Over half of the state schools deemed most socially selective by the researchers were those that pick pupils on the basis of faith.
Some 50 non-grammars appeared to be more academically selective than the least selective grammar school.
"How can it be that a non-selective school can have a 30 percentage point difference between the free school meals rate in the area from which it draws its intake and the rate for the pupils it actually enrols in the school," the report asked.
Half of these highly selective schools were in control of their admissions policies and half selected pupils on the basis of a faith, it added.
Even though they were not ostensibly academically selective, the report said some schools could pick some pupils by "aptitude" for a certain subject such as music, some were effectively "selecting by home postcode" when they became over-subscribed and many were faith schools.
Report author Dr Robert Coe said there was no evidence schools were deliberately selecting smarter pupils.
But he added: "There are incentives within the system for schools to take the nice pupils rather than the nasty pupils if you want to put it that way. We have to blame the league tables."
Faith schools, however, insist they tend to take a higher proportion of pupils from poor backgrounds.
The report also looked at the impact the academic selection of grammar schools had on other schools that the grammar pupils could have gone to.
In areas where there are large numbers of grammar schools, such as Kent, Medway, Buckinghamshire and Lincolnshire, many schools lose more than a fifth of their potential bright pupils to grammars.
Overall it found 35% of non-selective schools lose between 1% and 20% of the pupils they might have had to grammar schools.
A further 32% lose under 1%. This may partly be due to the larger area from which many grammar schools draw their pupils.
Many fill the available places with the pupils who score the highest marks in the entrance examination, paying no regard to where they live.
Nationally, some 20% of grammar school pupils come from outside the local authority of the school. But for some local authorities the figure is as high as 75%.
Dr Lee Elliot Major, director of research at the Sutton Trust, said the figures suggested that grammars were not enrolling as many academically able pupils from disadvantaged backgrounds as they could do.
"More spotlight needs to be focused on ensuring grammars do all they can to reach out to all potential pupils."
Chairman of the National Grammar Schools Association Robert McCartney rejected suggestions grammars were not taking enough poor, bright pupils.
He said: "You can only get into a grammar school if you subject yourself to an entrance test. Among deprived areas there is a dearth of parents pushing their children's educational aspirations."
A Department for Children, Schools and Family spokesman said it did not support academic selection at 11 and that parents could vote to abolish it in their area.
"For non-selective schools, the mandatory School Admissions Code gives children a fair and equal chance of getting into a school of their choice, regardless of background.
"It means all admission policies must be fair, clear and objective. We want parents to choose schools not schools to choose parents - and the large majority of schools have fair admissions."