The significance of grammar schools is "dramatically overplayed" and the system runs the risk of "social selection", according to an international education expert.
Andreas Schleicher, of the Organisation for Economic Co-operation and Development (OECD), said it would be best to make "most schools more demanding, more rigorous" and for them all to provide "better opportunities for disadvantaged children".
His remarks come after Labour leader Jeremy Corbyn warned school reforms desired by Theresa May were the sign of a government "heading backwards to a failed segregation for the few and second-class schooling for the many".
Mr Schleicher, who pointed out that the UK system was a lot less academically selective than many other countries, said he saw the case for making schools "more meritocratic" but added that he did not think more grammar schools would solve problems.
He said most performance variation in the UK did not lie between schools, but within schools, adding: "The problem of under-performance is not poor schools in poor neighbourhoods only. It's a lot of students in a lot of schools in a lot of neighbourhoods."
Mr Schleicher, who was asked questions about his views on grammar schools at a briefing on the OECD Education at a Glance 2016 report, said: "I think the importance of grammar schools is dramatically overplayed in terms of fostering academic talent."
He said "academic selection ultimately becomes social selection", adding: "Schools are very, very good in selecting students by their social background but they're not very good in selecting students by their academic potential.
"And the earlier they select the worse that relationship is.
"However, there are some countries very good at it. You go to Singapore, you go to Hong Kong. Those systems are almost truly meritocratic."
Earlier this week, a survey found that teachers and school leaders overwhelmingly oppose the Prime Minister's plans for a new wave of grammar schools.
Some eight out of 10 teachers and heads are against the selection proposals and also do not believe the tests taken at age 11 can measure academic potential, according to the research.
Mr Schleicher said tests were designed in such a way that "with enough money and effort" parents could get their children through them, adding: "Any kind of one-off assessment of students is likely to favour social background as opposed to true academic potential."
The expert said bright students in the UK did not always have the education opportunities they deserved and that many were probably not reaching their potential.
"But if I would want to address it I would look at what happens inside schools - the fact that too many students fall through the cracks within too many schools is a far bigger problem than having a few schools that are selective," he said.
Mr Schleicher added: "I can see the case for making the UK school system and English school system more meritocratic, more conducive to fostering high performance, all of this I can see.
"I'm not convinced that having a few more grammar schools is going to solve the problem, because the bulk of the issue lies within schools, not between schools."
Meanwhile, the Education at a Glance report found that in the UK children born to foreign parents were more likely to attain third level education than those with native-born parents.
The report also said that total expenditure (from public and private sources) on primary to third level education in the UK exceeded that of most OECD countries, especially at third level.