Plans for new grammar schools will mean the return of discredited secondary moderns as well, schools chief inspector Sir Michael Wilshaw warned today.

The outgoing head of Ofsted let rip at Theresa May’s decision to forge ahead with the controversial policy which will open the floodgates to many more grammar schools.

In her first domestic speech as Prime Minister today, Ms May will vow to reverse the ban on the creation of new grammar schools put in place by Tony Blair’s Government in 1998.

She will announce new selective schools will be allowed to open and existing schools given the opportunity to become grammars, insisting it will help in “making this country a true meritocracy - a country that works for everyone.”

In an interview today, Education Secretary Justine Greening – thought to be sceptical about the policy – argued only grammar schools could ensure pupils were “academically stretched”.

But Sir Michael, who is about to retire, told the BBC Radio 4 Today programme that the revolution would be “going backwards” and putting 20 years of progress in education policy at risk.

He said: “We will fail as a nation if we only get the top 15% to 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.”

Sir Michael warned, from his experience as a head teacher, of the damage that would be done to existing, successful all-ability schools if they lost their top 20% of pupils – thus turning them into old-style secondary moderns.

But Ms Greening hit back, insisting parents would get more choice, dismissing critics who say that, in selective schools, it is the school – not the parents – that does the choosing.

And she pointed out that schools are already able to select a proportion of their pupils on the basis of their aptitude in sport, music or the arts.

Ms Greening said: “This is not about going backwards, this is about a 21st century education system.

“Parents want choice for their children. There are some children that want and need to be academically stretched.

“This is not a return to secondary modern schools which, for a long period of time, did not even set an exam for children as they finished their education in them.”