Pupils from disadvantaged backgrounds in Buckinghamshire are more than a year behind their peers by the time they finish their GCSEs, new figures reveal.
The Education Policy Institute (EPI) has warned the figures indicate "a major setback for social mobility in our country".
In its annual report, the EPI found pupils on free school meals in Buckinghamshire were, on average, one year and three months behind their peers at the end of secondary school, a measure they call the 'disadvantage gap'.
This gap varies considerably across England, where poorer students are 18 months behind on average.
The EPI's annual report examines the progress made in closing the gap in educational attainment - the highest level of education achieved by an individual - for less well-off pupils, who typically get lower grades than their fellow students.
In Buckinghamshire, one in eight secondary school students are disadvantaged - eligible for free school meals - and just three per cent of pupils have been eligible for most of their school lives.
A Buckinghamshire County Council spokesman said: “The figures published this week show that Buckinghamshire pupils are performing strongly overall. The outcomes for disadvantaged pupils at the GCSE stage are amongst the 25 per cent most improved in the country and the gap between them and all other children is smaller than the national figure. The data for Buckinghamshire also showed a small improvement for children in Early Years settings whereas nationally the gap has increased.
“We are delighted our children from disadvantaged backgrounds are achieving higher than those nationally. This is due to the commitment from staff in schools to provide the right support to them. We are not complacent and will keep championing these children to make sure they have the best opportunities at every stage in their education.”
According to the institute, the disadvantage gap is "a leading indicator of how the government is performing on social mobility".
For the first time in several years, the difference in GCSE scores between poorer pupils and their peers has widened slightly, and the EPI estimates that it would take 560 years to close the gap altogether.
Persistently disadvantaged students in England, defined as pupils who have been eligible for free school meals for at least 80 per cent of their time at school, trail even further behind - almost two years on average.
David Laws, executive chairman of the EPI, said: "Educational inequality on this scale is bad for both social mobility and economic productivity, and this report should be a wake up call for our new prime minister.
"Recent progress on narrowing the education gap between poor children and the rest has ground to a halt, and we need a renewed evidence-based policy drive to change this."