At least 21 illegal schools are under investigation over fears of radicalisation it was revealed yesterday.
Inspectors believe up to 50,000 pupils may be having their minds filled with poison in unregistered schools.
Education chiefs are probing the institutions, scattered across England, following recent cases where pupils were being taught “narrow Islamic-focused curriculums”.
Inspectors also found institutions that were teaching hate-filled, misogynistic, homophobic and anti-Semitic material.
Last night Labour’s shadow education secretary Lucy Powell said ministers were taking too long to deal with the issue.
She said: “Despite warning after warning, they have dragged their feet, leaving children in unregistered schools where they could be in harm’s way.
“It is extremely worrying that Ofsted remains concerned that the number of children being educated in unregistered provision far exceeds the number currently known by the Government.”
Prime Minister David Cameron vowed to crack down on Islamic madrassas in his Tory conference speech last October. He insisted he would “shut down” schools that were “filling children’s head with poison and hearts with hate”.
Ofsted set up a task force in December to seek out unregistered schools operating outside the law in England.
At one in Birmingham, inspectors discovered squalid conditions and books full of “misogynistic, homophobic and anti-Semitic material”.
Last night a Department for Education spokesman said: “There will be no single knockout blow against those who seek to corrupt young people, but we are taking unprecedented and direct action across the board to protect children, inform parents and support teachers, putting us firmly on the front foot.”