Theresa May should stop “obsessing” about grammar schools and order a massive expansion of vocational education to address skills shortages that will worsen after Brexit, England’s chief inspector of schools says.

In a blistering attack on the prime minister’s priorities for education, Sir Michael Wilshaw, the head of Ofsted, says more grammar schools will “reduce standards for the great majority of children”, undo much of the progress of recent years, and be socially divisive.

Condemning the policy, reiterated by the prime minister at the Tory conference a fortnight ago, as a retrograde step, he says ministers should focus instead on promoting specialist, technical subjects in schools, and new technology colleges for 14-19-year-olds, so future generations have the skills necessary to drive the post-Brexit economy.

“If you’re going to make a success of Brexit, this is number one. It should be the number one government priority – not grammar schools,” he says in an interview with the Observer.

Wilshaw, who retires at the end of December after completing his five-year term, also turns May’s claim to want to govern for the “many, not the privileged few” against her.

“I came into teaching and I came into this job to raise standards for all children, not just for the few. And by their very nature grammar schools are for the few – otherwise why have them?” He says he is not “sitting on the fence on this one” but will speak out in the strongest way against a policy shift that will harm not only the education of young people but society too.

“It will actually lower standards for the great majority of children. That is my view. And it is socially divisive as well.”

The hard-hitting intervention from the man in charge of school standards in England is bound to fuel more division within the Tory party over May’s plans. Several Conservative MPs, including former education secretaries Nicky Morgan and Kenneth Clarke, have cited similar concerns to Wilshaw’s. In her party conference speech, however, May vowed to press ahead: “Where there is demand from parents, where they will definitely take pupils from all backgrounds, where they will play a part in improving the quality of all schools in their area – we will lift the ban on establishing new grammar schools.”

Ofsted chief says PM is wrong to focus on the grammar school issue. Photograph: Getty Images

Wilshaw responds by saying that the brightest pupils will be “siphoned off”, meaning that comprehensives are viewed in local communities as inferior. And he ridicules her claim that it will not lead to a return to a binary system. “What does that mean? If you’re taking away the best kids from the comprehensive system, you’re creating, by another name, secondary moderns. You can call it what you like. People will know that the brightest children, the most academic children, are not going there.”

Comprehensives have improved so much, he argues, that many now perform better than grammars. “If you had asked me 20 years ago whether comprehensives were working I would have said no. Ask me now and I would say in many parts of the country they are working extremely well. The latest research shows that the best comprehensives are doing better than grammar schools for the most able children.”

Another consequence will be that comprehensives will find it more difficult to recruit teachers. “Once you set up grammar schools and they’re seen as the high-status jobs, it’ll become much more difficult for other schools to recruit teachers. Look at Kent. Look at Buckingham.” He adds: “So rather than obsess about grammar schools, focus on whether we have enough good leaders who can lead systemic change, because that’s the direction of travel.”

He wants to persuade more youngsters that vocational education is a real alternative to university and believes a better-trained and more skilled workforce will be even more necessary after Brexit. “The skills shortages are getting worse and that will exacerbated, I suspect, by Brexit.”

He adds: “If I was secretary of state for education I would insist that every significant multi-academy trust contained a university technology college from 14 to 19 offering core subjects – English, maths, science – but also a specialism. And if you look at Germany and you look at Switzerland and you look at Norway, that’s the model that they pursue. They have a strong core but they also have specialist vocational schools, employer-led schools.”

The academy and free schools programme has, he says, given headteachers greater freedom and autonomy, allowing them to drive up standards, although he points out that some academy chains are still badly run and fail to intervene early enough in underperforming schools, replicating the failings of local education authorities.

The most difficult part of his job at Ofsted, he says, has been dealing with education ministers. “They come in, and they have got to make a difference. I understand that and obviously they’re looking for me – because I’m the bloke responsible for standards – to say lovely things about their policies, and that has been tough.”