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 Finland, Denmark, Germany and Canada, the gap is equal to 15 months.
The government in England says its reforms will improve reading standards.
And in Scotland, government officials say its new school curriculum is helping to raise standards.
The study was carried out by John Jerrim at the Institute of Education, University of London, who analysed scores for 15-year-olds in Pisa tests carried out for the Organisation for Economic Co-operation and Development (OECD).
He says the reading skills of England's youngsters as a whole are heavily linked to their socio-economic background, but that this gap is average for countries in the OECD.
However, he suggests when you look just at high achievers - the brightest in each socio-economic group - England and Scotland perform worse than other countries for boys and are "close to the bottom" for girls, out of the 32 nations included in the tests.
However, he warns there is some "uncertainty" around the rankings, because of variations in numbers of pupils taking part in the various countries and in sampling methods.
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.
The Sutton Trust is calling for better support for highly able children in state-funded schools in England.
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.
England's schools' watchdog, Ofsted, says its inspectors are to put more weight on how much progress is made by poorer children in schools.
Its head, Sir Michael Wilshaw, recently said "invisible" poor children were being let down by schools in leafy suburbs and coastal towns, while schools in many cities were closing the achievement gap.
A culture of low expectations meant bright pupils were not being stretched in some schools, he said.
The government in England is giving schools more funding for each pupil from a low-income home - about £600 per pupil now, rising to £900 in September. This is known as the Pupil Premium.
A spokeswoman for the Department for Education said the Sutton Trust 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.
"The new phonics check helps teachers identify children who are struggling with reading at an earlier stage and we are targeting more funding at disadvantaged children through the Pupil Premium," she added.
In Edinburgh, Minister for Learning, Science and Scotland's languages, Dr Alasdair Allan, said raising the attainment of all children was a key priority.
"An explicit part of this is closing the education gap and raising the attainment of children and young people suffering disadvantage," he said.
"Curriculum for excellence [the new curriculum] is helping to raise standards and means that literacy is now the responsibility of all teachers."
He added that a recent survey showed Scotland's schools were "achieving and sustaining a high performance in reading and writing skills" and that a new scheme was being developed where schools with good records on improvement partnered others to raise attainment.