Bright teenagers from poor backgrounds are more than two years behind pupils of the same age from well-off homes, according to research.
And this "huge gap" in achievement between bright children from poor and wealthy backgrounds in England and Scotland is twice as big as in some other developed countries, said study author Dr John Jerrim from the Institute of Education.
Dr Jerrim reached his conclusions after analysing the reading scores of 9,548 English and 2,631 Scottish pupils aged 15 who took part in the 2009 Programme for International Student Assessment (PISA) study. He then compared the scores of the top 10 per cent of children in five different socio-economic groups.
"I was essentially comparing the test scores of children whose parents are, for example, labourers to those of the children of judges, doctors and lawyers," he explained.
On average, the reading skills of poor, bright pupils in England were 2.5 years behind those of their clever well-off peers, while the attainment gap in Scotland was 2.75 years.
Compared with pupils of the same age in 23 developed countries, Scotland had the biggest attainment gap between bright children from advantaged and disadvantaged homes. England had a bigger attainment gap than 19 other countries.
In England the average gap in reading scores between advantaged and disadvantaged children – not just the high-achievers - was also about two-and-a-half years, but this is similar to that found in most other developed countries, such as Australia and Germany.
"Education policy over the last decade has focused considerable attention on improving the attainment of less able children from poor backgrounds, with some success," Dr Jerrim said.
"Now policymakers must turn their attention to reducing inequalities in educational achievement amongst the brightest children in society, to ensure that those from disadvantaged families are not left behind."
The study is published in a special issue of the Institute for Fiscal Studies journal, Fiscal Studies.