The most disadvantaged schoolchildren are two years behind their better off classmates by the time they sit their GCSEs, according to a new report.
It suggests that youngsters who have spent the majority of their time at secondary school eligible for free school dinners - a key measure of poverty - are increasingly lagging behind in their attainment.
The study, by the Education Policy Institute (EPI), also shows differences in the achievement gap between regions.
Researchers used government statistics for 2016 to examine the average performance of poorer pupils and that of their richer peers.
The findings show that across all state secondary schools, including special schools, "persistently disadvantaged" children - those eligible for free meals for 80% of their time at school - were 24.3 months behind their wealthier classmates.
Based on mainstream secondaries alone (excluding special schools), the gap has widened by 0.3 months since 2007, from 23.1 months to 23.4 months.
The report also shows the attainment gap between rich and poor pupils generally, for all types of state secondary school, now stands at 19.3 months.
When comparing children at mainstream secondaries alone, the report finds this gap narrowed by three months since 2007, from 21.9 months to 18.9 months in 2016.
From 2007 to 2016, the gap between rich and poor children at mainstream primary schools, by age 11, had narrowed by 2.8 months.
The researchers conclude that at the current rate of progress, it would take 50 years to reach a point where disadvantaged children did not fall behind their wealthier classmates during their formal education to age 16.
The findings also show that disadvantaged secondary-age pupils in the Isle of Wight are around 28.6 months behind their peers by the end of secondary school, while at the other end of the scale, the gap is 1.4 months in the London borough of Kensington and Chelsea, three months in Westminster and around seven months in the boroughs of Southwark, Wandsworth and Tower Hamlets.
Jo Hutchinson, the EPI's director for social mobility and vulnerable learners, said the research showed that the "most persistently disadvantaged pupils in England have fallen even further behind their peers".
"While some progress has been made with closing the gap for disadvantaged pupils overall, these pupils still fall behind at a rate of two months a year over secondary school," she said.
"At the current rate of progress it would take a full 50 years to reach an equitable school system where disadvantaged pupils did not fall further behind their peers between the ages of five and 16."
Shadow education secretary Angela Rayner said the findings were "deeply concerning but unfortunately come as no surprise".
"If you cut school budgets and push out teachers, as the Tories have done, then life in school will become even harder for the most disadvantaged kids," she said.
The leader of Isle of Wight Council, Dave Stewart, accepted children had been "badly let down" by the authority in the past.
He said: "We have set out in detail our school improvement plans in our recent publication delivering educational excellence and are determined that over the next four years all our schools will be good or outstanding.
"This is an ambitious target, but we are determined to deliver for all our children, including those who are disadvantaged."
A Department for Education spokeswoman said: "We are determined to ensure that all children, regardless of their background, get the excellent education they deserve."