The brightest boys from poor homes in England and Scotland are at least two-and-a-half years behind in reading compared with those from the richest homes, a study suggests.
Research for the Sutton Trust educational charity says Scotland's gap is the highest in the developed world, while England's is the second highest. In Germany and Canada the gap is equal to 15 months.
The government in England says its reforms will improve reading standards and the Scottish government says its new school curriculum is already helping to raise standards.
Study lead John Jerrim from the University of London said: "The reading skills of England's youngsters as a whole are heavily linked to their socio-economic background, but this gap is average for countries in the OECD.
"However, when you look at 'high achievers', England and Scotland perform worse than other countries for boys and are close to the bottom for girls."
Jerrim analysed scores for 15-year-olds in Pisa tests carried out for the Organisation for Economic Co-operation and Development (OECD).
In England, he says, the gap is equivalent to 30 months (two years and six months) of schooling for boys, while in Scotland the gap amounts to nearly three years (two years, 11 months).
Among the brightest girls, the gap in England is two years and four months.
After revealing these inequalities the Sutton Trust has urged state-funded schools in England to offer more support for highly able children.
The charity's chairman, Sir Peter Lampl, said: "By not stretching our most able students from all backgrounds, we are not only failing them, we are reducing our ability to compete globally.
"Moreover, such under-achievement perpetuates those inequalities which make it so hard for bright children to move up in society.
"That's why it is so important that there is a targeted scheme that ensures that those with high potential from low and middle income backgrounds are identified and helped to thrive," he added.
A spokeswoman for the Department for Education responded by saying the charity's report showed "the appalling attainment gap" that had been "a feature of our education system for far too long" and that government reforms would improve standards.