Summer-born children are more likely to be put into bottom groups for lessons than their older classmates, research suggests.
A new study has found that youngsters who were born in September, at the start of the school year, were three times more likely to be in top ability streams than those with birthdays in August, at the end of the school year.
There are also differences when pupils are grouped by ability within their own class, or for certain subjects, it says.
The report warns that grouping children in this way could be detrimental to younger children, who are already more likely to achieve lower results and be diagnosed with special educational needs.
The study, by the Institute of Education at the University of London, looked at information on the schooling of more than 5,000 children in England who are being followed as part of the Millennium cohort study.
It found that by the age of seven, just over 70 per cent of children born in September were in the top stream, compared with less than 30 per cent of those born in August. About 30 per cent of August-born children were in the bottom stream, compared with around 10 per cent of September-born youngsters.
The study found that 97 per cent of the children in the study were grouped by ability by the time they were seven – within their year, class, or both.
Around 30 per cent of the youngsters were grouped within their year for English, and 37 per cent were grouped for maths. Nearly four-fifths were grouped within their class for all or most lessons.
The study reveals that when children are split in class, summer-born pupils are more likely to be put into a lower ability group.
It suggests that around 55 per cent of September-born children who were grouped in class were put into a top ability set, compared with just over 20 per cent of those with August birthdays.