Theresa May's Jams are "significantly" less likely to send a child to a grammar school than their wealthier counterparts, raising concerns about the Government's plan to expand selective schools, an educational charity has said.
In areas where 10% or more of pupils attend the state-run selective schools, 15.8% of Year Seven pupils were judged to be from "just about managing" families, research by the Sutton Trust found.
In contrast, children from the wealthiest families made up 34.4% of Year Seven students at the schools, the charity found.
Sutton Trust chairman Sir Peter Lampl said: "Today's research raises concerns about the Government's plans to use new grammars as a vehicle for social mobility.
"We need to get existing grammars moving in the right direction before we consider expanding their number."
The charity's research split families into five groups based on income, with the lowest two groups judged to be Jams, and the wealthiest families occupying the top group.
When laying out her plans for new grammar schools in September, the Prime Minister said: "In a true meritocracy, we should not be apologetic about stretching the most academically able to the very highest standards of excellence."
But the Sutton Trust said its research suggested that even when children are similarly bright, the most economically disadvantaged are still under-represented in selective schools.
The data also found that among pupils eligible for free school meals, white children had the lowest rate of entry to grammar schools.
In 2016, 0.7% of disadvantaged White British children went to a grammar, compared with 0.9% of White non-British children, 0.8% of Black children, 2.8% of Indian children, 5.2% of Chinese children, and 1.5% of other Asian children.
The findings come after a major international report revealed that the UK's teenagers continue to lag behind their peers in countries such as Singapore, Japan, and Finland in maths, science and reading.
The study, published by the Organisation for Economic Co-operation and Development (OECD), ranked British children as 15th for science, 27th for maths and 22nd for reading, out of 72 countries and economies.
Higher attainment levels are part of the reason why parents like grammar schools, said the Sutton Trust report.
It found that in grammar schools in 2014/15, 97% of non-disadvantaged pupils and 93% of disadvantaged pupils achieved 5 A*-C grades at GCSE including English and Maths, compared to 84% of non-disadvantaged pupils and 64% of disadvantaged pupils at the country's top comprehensive schools.
However, the educational charity said this gap is likely to be because of the attainment levels of pupils upon entering the grammar schools, as opposed to the school itself.
The Sutton Trust report said: "There is a tentative consensus in the literature that, taking everything into account, there is likely a small benefit to attending grammar schools."
A Sutton Trust report in 2008 estimated this benefit to be between zero and three quarters of a GCSE grade.
In response to the findings, the Sutton Trust has called on the Government to make the admissions process for grammar schools fairer before their numbers are expanded.
They have suggested providing a minimum of 10 hours test preparation for all pupils to minimise the advantage offered by private tutoring, ensuring grammar schools prioritise pupils eligible for the pupil premium, and improving outreach work to disadvantaged families.
A Department for Education spokesman said: "The Sutton Trust itself has highlighted the positive impact grammar schools can have on pupils from less well-off backgrounds and that's exactly why we want more young people to benefit.
"Our proposals will address the issues highlighted in the report - creating more good school places in more communities and ensuring new and existing selective schools prioritise the admission of lower income pupils and support other schools to help drive up academic standards across the system.
"Thanks to the Government's reforms, almost 1.8 million more children are in schools that are rated good or outstanding than in 2010, but we are determined to go further so that all children, whatever their background, can go as far as their talents will take them."