
In a letter to Education Secretary Nicky Morgan, Wilshaw said there are likely many more unregistered schools to be found operating in England than have been identified so far.
“What we have found so far is likely to represent only a small proportion of the illegal schools operating across the country," he said.
"Inspectors are hearing about suspected new cases every week,” the letter stated.
“I therefore remain extremely concerned about the number of children and young people attending these schools who may be at significant risk of harm and indoctrination."
Wilshaw said that the evidence that inspectors have gathered so far during reinforces his belief that there are "many more children hidden away from view of the authorities in unregistered schools across the country than previously thought".
He warned that the schools are providing a sub-standard education, placing children at risk, and undermining the government's efforts to ensure that all schools are promoting British values, including tolerance and respect for others.
Wilshaw added that Ofsted's inspectors have been “deeply alarmed” by what they have found during some of these visits. He said serious fire hazards, unsafe and unhygienic premises, and staff and volunteers who have not been properly checked or cleared to work with children, have all been identified.
Wilshaw has previously been critical of government in relation to unregistered schools. Last November he said arrangements for closing down unregistered schools are "inadequate" and local authorities are not acting swiftly enough to identify them and ensure that all children and young people are kept safe.
The government, as part of its Counter-Extremism Strategy, has said it will develop a new system to intervene in unofficial Islamic schools, also known as madrassas, where there are concerns about the way children are being taught.
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