Ofsted is warning that pupils are being taught in “squalid” schools that are unregistered and unsupervised.
But Ofsted chief Sir Michael Wilshaw says the process to close such schools is “inadequate”.
Where there is such “illegal activity”, he says, the “full force of the law” should be brought to bear.
Sir Michael has written to the education secretary to say 15 such schools have been found and they should be “registered or closed down”.
He says there are “serious concerns” about the safety of children being taught in these “so-called schools”.
In response, Education Secretary Nicky Morgan says the government has taken “robust steps to tackle unregistered schools and improve safeguarding”.
“However we agree with Sir Michael that more needs to be done.”
As such Mrs Morgan says there are plans for “further powers to regulate settings which teach children intensively and to intervene and impose sanctions where there are safety or welfare concerns. We will be consulting on these proposals shortly”.
The Ofsted chief says inspectors visited 28 institutions where there were concerns and found 15 unregistered schools.
The schools that are still operating have not been identified, although some are believed to be in Birmingham and London. They include some linked to faith groups.
These are schools providing at least 20 hours a week of lessons, operating outside the supervision of the Department for Education, local authorities or Ofsted inspections.
Ofsted accuses the operators of some of these institutions of “using the freedoms afforded to genuine home educators as a cover for their activities”.
But Sir Michael is calling on the education secretary for greater urgency in closing unregistered schools and in the response of councils and the Department for Education.
And he warns against “bureaucracy, legislation or lack of resources” being used as a “reason for inaction”.
In one “deeply troubling case”, Ofsted says inspectors had to return for a fifth visit to Bordesley independent school in Birmingham, which inspectors now believe has closed.
Sir Michael says inspectors had been delayed from entering, but once inside had found “squalid conditions, including three single mattresses covered in filthy sheets in one room and no running water in the toilet areas”.
There was also “clear evidence of segregation, with separate classrooms for boys and girls” and “no evidence of appropriate vetting checks being carried out on staff”.
Inspectors also warned of “pupils being taught a narrow curriculum that was failing to prepare them for life in modern Britain”.
Sir Michael’s letter says there were subsequent questions about rights of access for the local authority – and as such “I arranged for police officers from the West Midlands force to accompany inspectors to facilitate entry to the premises”.
Mrs Morgan said: “We understand Bordesley has now closed and are keeping in close contact with Birmingham local authority as they work to urgently ensure the children involved are safe and are receiving suitable education.”
The Ofsted chief has told the education secretary: “The arrangements for closing down unregistered schools are inadequate.
“Too many children remain at significant risk of harm. I will continue to do all that I can to identify and inspect unregistered schools.”
Labour’s shadow education secretary, Lucy Powell, said: “It is hard to fathom why arrangements for closing down unregistered schools remain inadequate and swift action on this issue has not been taken.
“With children at risk of being exposed to harm, exploitation or undue influences, more inertia from this government simply isn’t good enough.”
Ms Powell warned of a “dangerous void” in the local oversight of schools.