An education boss is on collision course with Theresa May after he rubbished the Prime Minister's rumoured plans to approve a new wave of grammar schools.
Sir Michael Wilshaw, the Government's outgoing chief inspector of schools, dismissed the selective model - long favoured by many Conservatives - and said it would fail the poorest children.
Speaking at the London Councils education conference on Monday, Sir Michael said London pupils' attainment success - despite the lack of grammar schools in the capital - questioned claims that selective schooling would boost social mobility and help children from disadvantaged backgrounds.
He said: "If grammar schools are the great answer, why aren't there more of them in London?
"If they are such a good thing for poor children, then why are poor children here in the capital doing so much better than their counterparts in those parts of the country that operate selection?
"I appreciate that many grammar schools do a fine job in equipping their students with an excellent education. But we all know that their record of admitting children from non-middle-class backgrounds is pretty woeful."
According to latest figures from the National Grammar Schools Association, England has 164 state-funded fully selective schools, while Northern Ireland has 69.
The creation of new grammar schools was outlawed by the Tony Blair administration.
But reports last month suggested the Prime Minister was considering sanctioning around 20 institutions in mainly working class areas in an effort to improve social mobility.
The plans have been backed by several Conservative-linked pressure groups and think tanks.
But Sir Michael, who is due to leave his post after five years this autumn, said: "The notion that the poor stand to benefit from the return of grammar schools strikes me as quite palpable tosh and nonsense - and is very clearly refuted by the London experience."
The Prime Minister - a former grammar school student - refused to be drawn on the plans when questioned on the BBC's Andrew Marr Show on Sunday.
She said: "What I want to do in looking at schools is to build on the success of the six years under David Cameron's premiership where we've seen more children in 'good' and 'outstanding' schools. But there is still more to be done."
A Department for Education (DfE) said Education Secretary Justine Greening was "looking at the (grammar schools) issue".
Any move would be in contrast to Mr Cameron's reign, when he annoyed some Conservative backbenchers by resisting the creation of new grammar schools - instead focusing his education policy on academies and free schools which do not select on ability.
Opponents argue that the 11-plus exam, crucial in gaining a place at a grammar schools, led to elite institutions dominated by middle-class children while the majority of young people from poorer backgrounds received sub-standard education in secondary moderns.