The proportion of children on free school meals in inner London who obtained five or more A*-C grades at GCSE has more than doubled over a decade - but the gains were much smaller outside the capital, a report has found.
Researchers said the dramatic improvement in performance of disadvantaged children in London compared with those outside is due to steady improvements in the capital's schools, and suggested it could reflect its status as an economic powerhouse.
Another reason - accounting for around a sixth of the improvement relative to the rest of the country - is because London is much more ethnically diverse than the rest of England, and ethnic minorities tend to obtain better GCSE results than children from white British backgrounds.
The researchers found that less than a quarter (22%) of children on free school meals in inner London achieved the GCSE standard in 2002, but this had risen to almost half (48%) by 2013.
Outside London, the figure rose from 17% to 26%.
The report, compiled by researchers associated with the Centre for Analysis of Social Exclusion (CASE) at the London School of Economics (LSE) and the Institute for Fiscal Studies (IFS), concluded that the improved performance largely reflects gradual improvements in school quality.
They said improvements in primary schools played a major role in later boosts in secondary schools.
But they found the "London effect" for poor children began in the mid-1990s – well before many of the high-profile policies in secondary schools previously credited with the capital's success, such as the London Challenge, Teach First and the growth of academies.
Luke Sibieta of the IFS said: "London schools have become synonymous with educational success, particularly for poorer children. Our research shows that these improvements are not down to a single policy or factor.
"Instead, most of the improvements reflect gradual increases in the quality of schools stretching back to the mid-1990s. London's primary schools have become particularly successful and London's great secondary schools can then build on this success."
Researchers used detailed data from the Millennium Cohort Study to look at the performance of a large group of children born around the year 2000 and analysed their progress up to to age 11. They found that disadvantaged pupils in London are not ahead at the age of five, but make faster progress once they get to school compared with their peers outside the capital.
The main explanation for better GCSE results is that pupils from poorer backgrounds are entering secondary school with better test scores at age 11.
In 1997, about 47% of poorer pupils in inner London and the rest of England achieved the expected level in English tests at 11, but by 2008, poorer pupils in inner London became seven percentage points more likely to achieve the standard (75% for inner London compared with 68% for the rest of England).
The study, which also involved the University of Surrey, University of Bath and the UCL Institute for Education, said this points to a very important role for improved performance by primary schools in London, which was then built on in the capital's secondary schools.
Jo Blanden of the University of Surrey said: "London's schools have become extremely good at helping poor children succeed. This is despite the incredible diversity of their pupils.
"This success is likely to lead to better jobs and more social mobility among those educated in the capital."