Bright children from poor homes are more than two years behind their clever classmates from rich families, according to a new study.
It found that the gap in achievement between the very richest and the very poorest high performers is twice as big in England and Scotland than in other nations, such as Iceland, Finland and Germany.
The findings show that more must be done to help boost the results of the brightest children from disadvantaged backgrounds, report author Dr John Jerrim, of the Institute of Education, said.
He analysed the reading results of 9,548 English 15-year-olds and 2,631 Scottish students of the same age who took part in the Programme for International Student Assessment (PISA) study and looked at the scores of the top 10% of rich able pupils compared to the top 10% of poor able pupils.
The results show that on average, poor clever children in England are 2.5 years behind their richest high-achieving peers, while in Scotland the gap is 2.75 years.
This is twice as big as in Iceland, Germany and Finland, where there is a gap of around a year, the study found.
Overall, England had a bigger attainment gap than 19 other countries, while Scotland's gap was larger than 22 nations, putting it at the bottom of the table.
Dr Jerrim said he had essentially been comparing the test results of children whose parents are labourers with those whose parents are judges, doctors and lawyers.
"Education policy over the last decade has focused considerable attention on improving the attainment of less able children from poor backgrounds, with some success," 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 research is published today in a special edition of the Institute for Fiscal Studies journal, Fiscal Studies.
A separate study reveals that young people from the richest fifth of families are still almost three times more likely to go to university that the poorest fifth of pupils.
They are also nearly six times more likely to go to one of the UK's top, Russell Group universities.
The report, also published in Fiscal Studies, suggests that the difference is fuelled by the decisions pupils make about university applications, which is affected by exam results.
Students from disadvantaged families are less likely to have the right qualifications to apply, it suggests.