The chief of Ofsted has criticised Theresa May's plan to approve a new wave of selective grammar schools calling it "socially divisive".
Sir Michael Wilshaw says the move will lower standards for the majority of pupils and would only work "for the few".
The Prime Minister has repeatedly insisted she wants to create a country that "works for everyone, not just the privileged few".
But the education chief urged May to instead create specialist vocational schools led by employers and based on the models employed by the likes of Germany, Switzerland and Norway.
"If you're going to make a success of Brexit, this is number one," he told The Observer.
"I came into teaching and I came into this job to raise standards for all children, not just for the few.
"It will actually lower standards for the great majority of children. That is my view. And it is socially divisive as well."
The Government has promised there will be no return to the "11 plus" entrance exam for grammar school pupils and insists Britain already has a "postcode lottery", with richer parents moving to areas with better comprehensives.
But Sir Michael said: "If you're taking away the best kids from the comprehensive system, you're creating, by another name, secondary moderns. You can call it what you like. People will know that the brightest children, the most academic children, are not going there."
He also said grammars would inevitably make it more difficult to attract the brightest teachers to non-selective schools.
Several Tory MPs including former education secretary Nicky Morgan and Commons Education Committee chair Neil Carmichael oppose Mrs May's plan for new selective schools.