David Cameron has given the go-ahead to approve the opening of the first selective state school in England for 50 years, although the government’s decision to allow the new grammar school in Kent faces stiff legal and political opposition.

The Department for Education and education secretary Nicky Morgan are reported by the Times to have put the official seal on allowing the new grammar school to be opened in Sevenoaks, although Whitehall sources maintain that the ultimate decision after months of prevarication came from No 10.



The news instantly drew sharp criticism from Labour’s shadow education secretary Lucy Powell, who accused the Conservatives of sabotaging its efforts to use education to counteract the effects of disadvantage.



“Having made social mobility the centre of his conference speech, David Cameron should look at the clear evidence on grammar schools: they do not increase equality of opportunity, they make it worse,” said Powell.



“Tiny numbers of children from disadvantaged backgrounds pass their tests because they are the preserve of the privately tutored. This is a hugely backward step from a prime minister who we see once again saying one thing but doing the opposite.”



Labour was quick to send out a dossier of embarrassing statements by both Cameron and Morgan, including a comment by Morgan at last year’s Conservative party conference: “There aren’t going to be any more grammar schools under me … I am resistant to selective education.”



The issue of grammar schools has long been a faultline within the Tory party, although Michael Gove as education secretary had moved the party well away from supporting selection.

But since Gove’s demise the revival of grammar schools has been backed by senior figures, such as Theresa May and Boris Johnson.



Meanwhile, teaching unions and activists condemned the move, with several groups already preparing for a sustained legal battle over a possible breach of a law passed under Tony Blair that bans the opening of new selective state schools.



The new school for 450 pupils gets around the legal restrictions through a loophole, by being an expansion of the existing Weald of Kent girls’ grammar school. An earlier attempt to expand by the school was rejected by Gove as education secretary.



The official announcement will come in the form of a written ministerial statement to parliament on Thursday morning, although a spokesperson for the DfE refused to comment on the decision ahead of time.

The decision on the school’s revised expansion plan has been pending since last year but was delayed in the run-up to the election in May, and further delayed by Downing Street’s insistence that legal advice was robust enough to avoid any embarrassing failure in the courts.



At the heart of the legal opposition is the fact that the new site in Sevenoaks is nearly nine miles from the site of Weald of Kent school in Tonbridge. The Times reports that the school will attempt to get around the legal objections by bussing pupils and staff between the two sites on a weekly basis.



Paul Carter, the Conservative leader of Kent county council – which backed the move, said: “The school took great effort to submit their bid to give greater detailed information on how it will be one school, not two schools, and they did a very good job.”



If the new school survives the expected judicial review, a flood of similar applications to open expansion sites is expected, although the loophole is only possible in local authorities and boroughs that have maintained grammar schools since their attempted abolition in the 1970s.



Any expansion of selective schooling – where pupils are selected at the age of 11 through examination – comes in the face of research that contradicts the idea of grammar schools as vehicles of social mobility, and have instead been colonised by the middle class.



Existing grammar schools have very few pupils from families that qualify for free school meals – in most cases a household income of £16,000 or less – while the schools themselves have struggled to come up with selection exams that avoid favouring those with access to private tuition.



Critics of selective schools include the Ofsted chief inspector Sir Michael Wilshaw, a former state secondary school head teacher.



“Grammar schools are stuffed full of middle-class kids. A tiny percentage are on free school meals: 3%. That is a nonsense,” said Wilshaw in an interview with the Observer.



“Anyone who thinks grammar schools are going to increase social mobility needs to look at those figures. I don’t think they work.”



Labour also highlighted a comment by Cameron in the Guardian from 2007: “Parents fundamentally don’t want their children divided into sheep and goats at the age of 11.”