British schools deserve to score only 6.5 out of 10 compared with education systems in other countries and would be further set back if new grammar schools were allowed, the chief inspector of schools has said.

Sir Michael Wilshaw, head of Ofsted, said the UK education system was getting better since it had been in “special measures, in intensive care, in the 70s, 80s and much of the 90s”, but an expansion of grammar schools planned by Theresa May would stall progress.

Wilshaw has spoken out several times against new grammars since the policy was announced by the prime minister. May wants schools to be able to select on academic achievement if they meet conditions about helping children from poorer backgrounds.

The Ofsted chief told BBC1’s Andrew Marr Show on Sunday: “If you have grammar schools, you take away the most able children from the all-ability comprehensive set-up. And I speak as an ex-head of a successful inner-city comprehensive school, an academy, in Hackney. I needed those top 20% to lift everyone up.”

He said the UK system had a long way to go before it could match the achievements of South Korea, Shanghai, and some European countries. “It’s six and a half out of 10,” Wilshaw said. “Mediocre, but getting better.”

He said he would not have been able to turn around his academy school in east London if there had been a nearby grammar.

“I wouldn’t have been able to do it if those top, those most able youngsters went to the grammar school, because it was their ambition, their excellence that affected the rest of the school community,” he said. “And we need now more youngsters – in the sort of economy that we’ve got, the service, digital economy – to do well academically.”

Lucy Powell, the former shadow education secretary, said Wilshaw was right that education had vastly improved over the past 20 years, but added that grammars would harm – not help – when there is a “long-tail of underachievement”.