Ofsted chief inspector Sir Michael Wilshaw has described the idea that a new wave of grammar schools would help the poor as "palpable tosh and nonsense".

The Government is widely expected to announce plans to allow more schools to academically select in the coming weeks, with social mobility likely to be a key justification for the policy.

But in a speech this morning Sir Michael said: "​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."

The chief inspector highlighted the improvements of school standards in inner London and said: "If grammar schools are the great answer, why aren’t there more of them in London?"

He also pointed to the mix of the pupils in "the small number of London boroughs that have grammar schools".

"The proportion of disadvantaged children in the four grammar schools in Bexley, for example, is around 9 per cent compared to 28 per cent in its non-selective schools. Similarly in Sutton, where five out of the 14 schools are grammar schools, the proportions are about 7 per cent compared to 25 per cent.

"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."

His comments follow an interview on Saturday when Sir Michael also condemned the idea of more grammar schools.

Speaking to the Times, the chief inspector said: "I think we will go backwards if we return to a system where we only expect some kids to do well. Every time you create a grammar school you create three secondary moderns and no one is queueing up to go to a secondary modern.”

The government's plans first emerged last month and have faced a storm of criticism from the education sector.

Sir Michael, who led two successful comprehensives in East London before taking up the job at Ofsted, went on: "If someone had opened up a grammar school next to Mossbourne Academy, I would have been absolutely furious as it would have taken away those youngsters who set the tone of the school. Youngsters learn from other youngsters and see their ambition; it percolates throughout the school.

"That is why we sent so many children to great universities. If the grammar schools had siphoned off those top 20 per cent, we would have floundered.”

"In Kent [which has selection] the gap in attainment between free-school-meals and non-free-school-meals pupils at key stage 4 is 34 points. If you go to inner London where there are no grammar schools, the gap is 14 points so it is socially divisive.”

Sir Michael used his Saturday interview to attack mixed-ability teaching, speaking out in favour of setting. "Mixed ability has also been a failure," he said. "I don’t care what anyone says, it rarely works. It didn’t operate in the schools I led."

He also revealed that: