Schools in affluent areas are failing their poorest pupils, according to new research.
Disadvantaged youngsters are more likely to lag behind in schools where they are in the "extreme minority".
The new study, published by the Education Policy Institute (EPI), found there are wide differences in the progress poorer pupils make in English and maths, depending on where they are educated.
EPI chairman and former education minister David Laws said the findings suggest that there is still a "mountain to climb" to close the gap between rich and poor pupils.
Researchers analysed the progress different groups of children are making at certain points in their schooling.
Overall, the progress gap is closing fastest in schools where disadvantaged youngsters make up more than 45% of pupils.
At Key Stage 2 (age seven to 11) these schools have eliminated the progress gap in reading and maths completely over the last decade, meaning that poorer youngsters - those eligible for the pupil premium - make the same level of progress as their classmates do nationally.
In comparison, primaries with the lowest proportions of disadvantaged youngsters (less than eight per cent) have seen the gulf widen.
"Since 2006, the Key Stage 2 progress gap in these schools has increased from 1.1 to 2.6 months," the study says.
"This growth has levelled off since 2012, but nonetheless paints a worrying picture of the prioritisation and focus that disadvantaged children are given when they are in the extreme minority."
The study sets out a number of recommendations, including suggesting that more should be done to make schools where poor pupils are in the minority accountable for the progress of these youngsters.
Mr Laws said: "The Education Policy Institute has today published the first comprehensive analysis of how our education system has supported the disadvantaged over the last decade. The conclusions are mixed - with schools in poorer areas doing well but in affluent areas failing to close gap.
"With the new prime minister in her first speech making a commitment to improving life chances, this research is timely and indicates the mountain to climb in terms of the gap between disadvantaged pupils and the rest of pupils."
Separate research published by the Sutton Trust today argues that each year, thousands of poorer pupils are missing out on taking languages and humanities subjects at GCSE.
The study looks at the impact of the English Baccalaureate (EBacc) a measure introduced by the coalition government that recognises pupils who score at least a C at GCSE in English, maths, science, history or geography and a language.
Overall, there are around 15,000 poorer students who should be expected to take a humanities subject at GCSE, but do not, and around 11,000 who should be taking a language, it concludes.
A Department for Education spokeswoman said: "As the Prime Minister set out this week, opportunity should not be restricted to a privileged few and every child should be afforded the chance to fulfil their potential - and that starts with an excellent education, no matter what their background.
"We are making progress with over 1.4 million more children in good or outstanding schools than in 2010 and the attainment gap between disadvantaged pupils and their peers is falling at primary and secondary level."
On the Sutton Trust report, she added: "As this report highlights, the EBacc is already helping children, particularly those from disadvantaged backgrounds, to benefit from a rigorous education.
"The take-up of history, geography and languages indicates that the English Baccalaureate is actively increasing the uptake of these subjects."