Ofsted is to set up a taskforce of inspectors to seek out unregistered schools that are operating outside the law in England.
Chief Inspector of Schools Sir Michael Wilshaw says there is a serious and growing threat to the safety of children.
Inspectors have found three unregistered schools operating in Birmingham in recent weeks.
The government says it is taking robust steps against unregistered schools.
Analysis: From BBC education editor Branwen Jeffreys
Anywhere that offers more than 20 hours a week of teaching is legally obliged to be registered as a school. The three schools inspected by Ofsted had not met this requirement to register.
It is likely the children being sent to some of the centres breaching that threshold are registered as home-educated.
The three places visited by Ofsted at the end of November bring the total of unregistered education centres inspected in recent months to 18.
Of these, the only one to have been named is Bordesley Independent School, in Birmingham, where inspectors found dirty mattresses and a lack of running water.
Ofsted says it found a narrow Islam-focused curriculum in the latest schools it investigated.
In one, conditions were described as unhygienic and filthy, with staff who had not been screened for their suitability to work with children.
Inspectors also found books described as inappropriate, with texts including “misogynistic, homophobic and anti-Semitic material”.
In the three Birmingham settings, 94 children were being educated.
Writing formally to the education secretary today, Sir Michael says he believes the number of children in unregistered schools could be far higher.
He warns that Ofsted’s work to promote British values is being undermined by the growth of these unregistered schools.
The chief inspector says he is also concerned that some of these education centres have received what could be “confusing and unhelpful” advice from Department for Education officials that they can continue to operate while applying to register.
He says it sends out “entirely the wrong message” and could have the unintended consequence of encouraging others to open such schools.
While most of the places visited by Ofsted so far have been in Muslim communities, inspectors may also turn their attention to similar unregistered tuition centres operated by other groups.
“It will apply across the board, to any religious group whether Muslim group, Jewish group, or Christian group who wants to operate this sort of provision in unsafe accommodation, in unhygienic and filthy accommodation.
“It will apply to all religious groups, I want to make that absolutely clear,” Sir Michael told the BBC.
Ofsted says it wants to work with officials to disrupt and close down illegally operating schools and intends to prosecute suspected proprietors.
A Department for Education spokesman said the government had been taking robust steps to tackle unregistered schools and improve safeguarding since 2010.
“This includes being completely clear it is a criminal offence to operate an unregistered independent school.
“We agree with Ofsted that more needs to be done and will be strengthening our communications to potential providers.
“We are also introducing further powers to regulate settings which teach children intensively and to intervene and impose sanctions where there are safety or welfare concerns.
“We are pleased Ofsted has agreed to take forward these prosecutions and we look forward to immediate progress.”
Shadow education secretary Lucy Powell said the government had “failed to take swift action” to improve arrangements to close unregistered schools “despite warning after warning”.
“The dangerous void in the local oversight of our schools system, created by this government’s education policy, is allowing children to end up in harm’s way,” said Ms Powell.
“That this government appears to be content to allow children to remain in illegal schools where they are at risk of exposure to narrow curriculums, misogynist, homophobic and anti-Semitic material and staff who haven’t been cleared to work with children, is truly shocking,” she added.
“Every day children remain in such a school is a day too long.”
A spokesman for Birmingham City Council said the authority was continuing to work with Ofsted and the DfE “to keep our children safe”.
The council welcomed “the focus on this issue which we highlighted earlier this year”.
“However, the DfE as the regulatory body needs to use its powers to deal with these places that are denying children a proper education, close them down and ensure those responsible are prosecuted,” said the spokesman.
The British Humanist Association said it was glad the problem of unregistered schools, “the majority of which we know are religious, finally seems to be being taken seriously, especially after successive governments have known about them for so long.
“Every year that these schools are either allowed to stay open, or else are able to evade detection, thousands more children are subjected to indoctrination, to abuse, and to an education so limited and blinkered that it can scarcely be described as an education at all.”
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- ‘Radicalisation risk’ at six Muslim private schools, says Ofsted 21 November 2014
source: BBC NEWS