The chief inspector of schools has warned that Theresa May's plan to bring back grammars threatens up to 20 years of progress in the education system.
Ofsted head Sir Michael Wilshaw said dividing the school system could affect the UK's ability to compete with the rest of the world.
The Prime Minister is due to announce a wave of new grammar schools in England in a bid to end "selection by house price" and give every child the chance to go to a good school.
Selective schools will have to meet targets on how many places they offer pupils from poor families or set up new open access schools and help failing institutions.
But Sir Michael, who is set to retire, warned against Mrs May's controversial reforms.
He told the BBC Radio 4 Today programme: "My message is there's been a turn-around in state education because of reforms and we don't want to put that at risk.
"A good comprehensive, a good all-ability school can deliver for the most able children and we're seeing more of that now."
He added: "We will fail as a nation if we only get the top 15%-20% of our children achieving well.
"We've got to, if we're going to compete with the best in the world, get many more children to achieve well in our schools.
"My fear is that by dividing children at 11 and by creating grammars and secondary moderns - because that's what we'll do - that we won't be able to achieve that ambition."
He added that the overall culture of all-ability schools would be negatively impacted if they lost their top 20% of pupils.
In her first domestic speech since entering No 10, Mrs May will claim that back-door selection by postcode and wealth is "unfair".
The premier will dismiss Labour's opposition to selection as sacrificing children's potential for "dogma and ideology".
Mrs May will say: "We are going to build a country that works for everyone, not just the privileged few. A fundamental part of that is having schools that give every child the best start in life, regardless of their background.
"For too long we have tolerated a system that contains an arbitrary rule preventing selective schools from being established - sacrificing children's potential because of dogma and ideology.
"The truth is that we already have selection in our school system - and it's selection by house price, selection by wealth. That is simply unfair.
"That is why I am announcing an ambitious package of education reforms to ensure that every child has the chance to go to a good school.
"As well as allowing new selective schools, we will bring forward a new requirement that means universities who want to charge higher fees will be required to establish a new school or sponsor an existing under-performing school.
"This is about being unapologetic for our belief in social mobility and making this country a true meritocracy - a country that works for everyone."
Mrs May told the 1922 Committee of backbench Conservatives on Wednesday that the current state school system allows wealthier parents to ensure a place for their children at high-performing schools by buying homes in the catchment area.
But the Government's social mobility tsar, Labour former Cabinet minister Alan Milburn, said grammars could be "a social mobility disaster".
Labour shadow Angela Rayner warned MPs that an expansion of grammar schools would "entrench inequality and disadvantage".
Rules stopping faith schools from selecting more than half of pupils according to religion are also set to be relaxed under the proposals.
A Downing Street source said: "The admissions cap had the best of intentions but it has failed in its two key tests. It has failed to make minority faith schools more diverse, because parents of other religions and none do not send their children to those schools.
"But it has prevented new Catholic schools from opening, which are more successful, more popular and more ethnically diverse than other types of state school.
"We're going to change the rule, so we can allow new Catholic schools to open, while making faith schools of all kinds do more to make sure their pupils integrate with children of other backgrounds."