Many more children than previously thought are being "hidden away" from view in illegal schools, Ofsted’s chief inspector Sir Michael Wilshaw has warned.
In a letter to education secretary Nicky Morgan today, Sir Michael states that his inspectors have indentified 100 unregistered schools, including seven in the last month alone.
And the former headteacher has warned that the schools are "exploiting weaknesses" in the legislation that allows parents to home educate their children.
"I have previously voiced concern that many of those operating unregistered schools are unscrupulously using the freedoms that parents have to home educate their children as a cover for their activities," Sir Michael writes.
"They are exploiting weaknesses in the current legislation to operate on the cusp of the law. Many are charging thousands of pounds to send their children to these unregistered schools."
Undermining British values
The schools, he adds, are offering a "substandard education", placing children at risk and "undermining the government’s efforts" to promote British values.
"I know we both share a determination to take swift and decisive action against those who are operating illegal schools and thereby putting children at risk of harm, including the risk of exposure to extremism and radicalisation," Sir Michael states.
The chief inspector has previously written of his concerns about illegal schools, and has asked a group of inspectors to work on the issue.
He said that the group remained "deeply alarmed" by what they had found, including cases where children were eating their lunch near chemicals used in chemistry lessons, and staff and volunteers who had not been through the proper checks.
Last month, Ms Morgan told a select committee hearing that she would be looking into introducing more regulation around home educated children.