Children of 'just managing families left out by grammars'
Lack of access to grammar schools is not confined to the poorest children, those from "just managing" families are also left out, research suggests.
"There is a strong indication that families on below average earnings are not being helped by the current grammar school system," said the Sutton Trust.
Grammar schools in England should not expand until the government can ensure fair admissions, the charity argues.
Ministers said their plans would address these issues.
The government's consultation on proposals to lift the ban on opening new grammar schools ends on Monday.
Prime Minister Theresa May has insisted that the plan will not represent a return to "the system of binary education from the 1950s" with a grammar in every town.
The Sutton Trust looked at entry to selective schools in 2015, according to neighbourhood deprivation.
In selective education areas outside London, the researchers found a third (34%) of Year 7 pupils were from the richest neighbourhoods while only 4% came from the poorest and 11% were from neighbourhoods with below average incomes.