Introducing more grammar schools in England is unlikely to improve social mobility, OECD education director Andreas Schleicher has said.

Mr Schleicher, who runs the influential Pisa (Programme for International Student Assessment) international rankings, told a press conference at London that in most European countries “academic selection ultimately becomes social selection”.

And he added: “Schools are very, very good at selecting students by their social background but they are not very good at selecting pupils by their academic potential.”

Theresa May set out her aims for the policy last week, saying that selective schools would be a way for children from ordinary, working-class families to be given the chances their richer contemporaries took for granted - and adding that new grammars would need to demonstrate how they will attract pupils from different backgrounds. 

But the proposals have been drawn widespread criticism, with high-profile education figures such as Sir Michael Wilshaw, the outgoing chief inspector, saying the idea they would help the poor was “palpable tosh and nonsense” and former education secretary Nicky Morgan saying they could undermine the Conservatives’ reforms. A TES survey of more than 1,100 teaching staff found 72 per cent did not think it was a good idea to have more grammar schools.

Mr Schleicher said that he could understand the case for more meritocracy in the UK school system, saying that “many students are not properly reaching their potential”.

He also added that in some countries, such as Singapore, there were selective systems which worked.

But he added: “But in our [European] countries it seems to work the opposite way, basically our schools are not very good at identifying true academic potential.”

He said that to address the problem of underperforming students in England, he would recommend looking at what happens within schools - not between them.

“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.

He was speaking at a press conference in London to launch the OECD Education at a Glance 2016 an annual report, which shows how education systems around the world compare.

The report revealed that children born to foreign parents in the UK are more likely to get a degree than those with British-born parents, in contrast to most developed countries.

It also revealed that the UK has among the youngest teaching workforce of all developed countries with 22 per cent of teachers aged 30 or younger compared to an average across the OECD of nine per cent aged 30 or younger.

And that teacher pay has decreased between 2010 and 2014, meaning starting salaries for teachers in England and Scotland were comparatively low at the start and end of their careers - although above the OECD average after ten years of service.

“In the UK teachers have really suffered,” said Mr Schleicher. “When you look at most countries teachers’ salaries have risen in the last couple of years, but in UK we see a downward decline in teacher pay.”

A Department for Education spokesperson said: “Every child, regardless of background or ability, should have access to an excellent education.

“We know that grammar schools provide a good education for their disadvantaged pupils and we want more pupils from lower income backgrounds to benefit from that.

“Our proposals will ensure that any new and existing selective schools will prioritise the admission of disadvantaged pupils and that they support other local pupils in non-selective schools to help drive up educational outcomes. As set out in the consultation document, we are clear that relaxing restrictions on selective education can and should be to the betterment, not at the expense, of other local schools.”