Grammar schools are dominated by children from well-off and middle class families, according to figures released by the government in its effort to show that “ordinary working families” would benefit most.

Education experts said data published by the Department for Education (DfE) demonstrated that the children of the highest earners were more likely than not to win places at selective schools.

Rebecca Allen, the director of the Education Datalab thinktank, said new DfE figures only confirmed previous research that children in the top 10% of households in terms of income were the most likely group to gain entrance to grammar schools via academic selection.

“It is only the very wealthiest families that are more likely to find their children in a grammar school than in a secondary modern,” Allen said.

The paper attempted to define who “ordinary working families” were: households with incomes of up to £33,000 a year.

Allen added that the DfE statistics backed up academic research published earlier this year that found children from households in the top 1% of income enjoyed an 80% chance of admission to selective schools.

Tables published in the DfE’s technical consultation paper showed that children from households above median income took more than half of existing grammar school places, despite accounting for only a third of the school-age population in England.

“I don’t think it is particularly helpful to use statistics as the government has, because it seems to imply that low income families stand as good a chance of gaining access to a grammar school as they do a comprehensive,” Allen said.

Analysts from the Education Policy Institute said: “Grammar schools are dominated by the most affluent, squeezing out the poorest. An expansion in selection is unlikely to benefit [ordinary working families] in the way that the government suggests.”

In a keynote speech on Thursday the education secretary, Justine Greening, used the analysis to justify removing the bar on opening new grammar schools in England, and warned that children from working class families had gone “off the radar” of politicians.

“We believe it’s not just disadvantaged children and young people that our education system can deliver much more for. This government will not lose sight of other children, from ordinary working families.

“This government believes we have not done enough to support them – partly because they do not qualify under our existing measures of disadvantage,” she said.

Responding to questions after her speech, Greening gave an “absolute assurance” that disadvantaged children would not be overlooked by the government.

But Allen warned the DfE figures showed that the starkest divide in school outcomes was in the exam results of those on free school meals, which were well behind those from “ordinary working class” households.

“Those who have not claimed benefits during secondary school typically do not have children who are struggling educationally, even if they themselves are on very low incomes,” Allen said.

The DfE analysis relied on parents’ tax and benefits data, linked to the national pupil database, and did not take into account a family’s self-employed earnings or capital assets, or the social or economic backgrounds of parents.

“If we want to create useful indicators that tell us how much the state should intervene to support a child at risk of falling behind at school then we’ll have to do better than simply matching tax records to the national pupil database,” Allen said.

For Labour, the shadow education secretary, Angela Rayner, said the government’s new definition of “ordinary working families” was a way of hiding the full picture. “In effect, they have not been able to find the evidence to back up their ideological policy, so they’ve created some themselves, they’ve cooked the books,” Rayner said.

“They are trying to fiddle the numbers – even in their own research they show 53% of the wealthier than average families are going into grammar schools, as opposed to 20% in the comprehensive system. So even [in] their own cooking of the books they cannot hide from the fact that grammar schools do not aid social mobility.”