Pupils from disadvantaged backgrounds in Calderdale are almost a year and a half behind their peers by the time they finish their GCSEs, new figures reveal.
The Education Policy Institute has warned that the figures indicate "a major setback for social mobility in our country".
In its annual report, the EPI found that pupils on free school meals in Calderdale were, on average, one year and five 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.
Five local authorities reported a gap of two years, while in Westminster the difference is just four months.
The EPI's annual report examines the progress made in closing the gap in educational attainment for less well-off pupils, who typically get lower grades than their fellow students.
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.
In Calderdale, more than a quarter of secondary school students are disadvantaged - eligible for free school meals - and around one in 10 pupils have been eligible for most of their school lives.
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."
Angela Rayner, shadow secretary of state for education, blamed the Government for squeezing school budgets, and promised that Labour would "end Tory cuts".
She added: "Sadly, there is no reason to expect that will change with the new Prime Minister and Education Secretary, who are intent on handing out yet more massive tax giveaways to the super-rich rather than investing in all our children."
Responding to the EPI's annual report, school standards minister Nick Gibb said that the disadvantage gap had "narrowed considerably" since 2011.
He said: "During that time, this government has delivered a range of reforms to ensure that every child, regardless of their background, gets a high-quality education."