Poor pupils are not allocated priority places at more than half of England's grammar schools, research suggests.
Analysis by the BBC shows that 90 of the 163 grammar schools' admissions policies do not consider a child's eligibility for free school meals.
Government plans to increase the number of grammar schools has seen them come under increasing pressure to become more socially inclusive.
According to the broadcaster, the most recently published admission policies for applications for 2017-18 reveal the extent to which change is under way.
Pupils are required to pass entry exams to gain a place at the schools and, if oversubscribed, the institutions allocate places according to their admission policies.
The research revealed that 21 grammar schools set aside places in quotas for pupils from lower income families.
Nine grammar schools use it to decide between academically matched pupils, while a further 33 give some degree of priority in their oversubscription criteria.
Professor Anna Vignoles, from the University of Cambridge, described the research as "insightful", adding: "Whilst quotas or other measures might help, the fundamental problem in the system is the very large gap in achievement between free school meals pupils, and others, at the end of primary school."
The Government has promised there will be no return to the 11 Plus entrance exam for grammar school pupils and insists Britain already has a "postcode lottery", with richer parents moving to areas with better comprehensives.
Prime Minister Theresa May also stressed that new grammars will have to show they are "genuinely reaching out" to poorer pupils and that education is "not going back to the 1950s".
BBC analysis suggests no clear correlation between admission policies and the level of deprivation within a few miles of the school's location.
A Department for Education spokesman said: "The Government is currently consulting on scrapping the ban on new grammar schools, and allowing them to open where parents want them. Too often, the chance of getting the best education depends not on their talent or hard work but on where they live or how much money their parents have.
"Our new approach is not about recreating the binary system of the past or maintaining the status quo. We want to look at how we can ensure new selective schools prioritise the admission of pupils from lower income households and support other local pupils in non-selective schools to help raise standards."