The cat is well and truly out of the bag. After David Cameron managed to hold off his right-wing backbenches for almost 10 years, it would seem the new Prime Minister has not even lasted the first few months.

She appears to have accepted the assumption that new grammar schools will improve social mobility, and help poorer children out of a cycle of perpetual disadvantage. But we have 70 years of evidence now, which demonstrates the exact opposite.

We have only learnt about this through reports of secret briefings to Tory MPs and documents photographed on the way in to Number 10. The Education Secretary even tried to deny today that there was any policy at all, despite the Prime Minister’s quoted words and the Cabinet paper that we have all read in the national media. From the way the Tories have gone about this, you would think they were actually quite embarrassed – and they should be.

Children from disadvantaged backgrounds, including the white-working class boys that the Prime Minister spoke about on the steps of Downing Street when she took office, are not the children that stand to benefit from these new grammar schools.

In fact poorer children in local authorities that currently operate a grammar school system perform worse than those in non-selective authorities, and are far less likely to score highly at GCSE.

Far from aiding social mobility, this policy will just create social division. According to the Sutton Trust, only three per cent of pupils in grammar schools are entitled to free school meals, compared with an average of 15 per cent of children in receipt of free school meals across all state schools.

This segregation affects these children for the rest of their lives. The Institute for Education found that the wage gap between the highest and lowest earners in selective local authorities is higher than in non-selective areas.

In the perceived “golden-age” of the grammar system in the 50s, which Tory backbenchers seem to look back to through rose-tinted glasses, almost 40 per cent of the children selected failed to achieve more than 3 O-Levels. This system failed to provide Britain with the dynamic, skilled workforce that we still need today.

The claim that they are “fundamental to social mobility” just does not stand up. The truth is that those children at the remaining grammar schools today are overwhelmingly the lucky few whose parents can afford tuition for the 11 Plus – and the Sutton Trust has recently revealed that a fifth of paid tuition in England and Wales is for the grammar school entrance exam.

Sir Michael Wilshaw pointed out this week that in Hackney the attainment gap at GCSE level between the most affluent and most disadvantaged pupils is 14 per cent. In Kent, one of the few counties that still operates a grammar school system, it is 34 per cent.

And nobody in the government who supports new grammar schools has been honest enough to talk about new secondary moderns, the natural consequence of allowing new grammar schools to change the intakes of existing local comprehensives.

The Prime Minister has said that her approach to policy will be to assemble the evidence, look at the evidence, and then come to a decision. That is the test she has set herself. To push forward with new grammar schools as the centre piece of her education policy would fail that test spectacularly.

She has pledged to make Britain a country that works for everyone, and give everyone the opportunity to go as far as their talents will take them.

But far from promoting social mobility, the government has actually overseen an education system in which the attainment gap between disadvantaged children and their peers is now widening.

New grammar schools are not a solution to that. At best they are just a distraction from it.

The government are not so keen to address what existing schools are facing right now. There are chronic shortages of teachers, after years of missed government targets for recruiting new trainees and demoralising the profession. The number of teachers quitting – some 50,000 last year – is at a record high.

If the Prime Minister is serious about improving social mobility, she would do well to listen to her predecessor who warned his backbenches almost a decade ago that their obsession with grammar schools was “splashing around in the shallow end of the educational debate.”

The education debate has moved on, and it is time the Tories did too.