A study of examination results has indicated that the top pupils in comprehensive schools do just as well at GCSE as their peers in grammar schools.
And it suggests that those performing just below the level of the brightest pupils perform better at comprehensives.
The research has been carried out by Professor David Jesson, a consultant in educational policy and performance at the University of Sheffield.
It is based on a comparison of results pupils receive at the ages of 14 and 16.
It also suggests that the impact of grammar schools reduces the overall level of performance at GCSE in local education authorities where grammars exist in substantial numbers.
This is because scores are lowered by the poorer performance of children at secondary modern schools, according to Professor Jesson.
'Degree of complacency'
The findings were revealed at a conference held by the Campaign for State Education (CASE) organisation, which is trying to abolish selective education.
Professor Jesson said the number of pupils gaining five or more A* to C grades at GCSE would increase by 3% if all schools were open to all pupils.
"Three-quarters of kids in grammar schools would do better in comprehensive schools," he said.
"I think there is a degree of complacency concerning the middle-ranging youngsters in grammar schools, which is perhaps not there as much in comprehensives.
"For every selective school there is a secondary modern, which does depress results."
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