Inspectors are investigating at least 21 unregistered schools over fears they are radicalising students with “narrow Islamic-focused curriculums”.

The “illegal” schools are under investigation after allegedly teaching extremist material to up to 50,000 children across Britain.

Labour’s Shadow Education Secretary Lucy Powell accused ministers of taking too long to deal with the issue. She told The Sun: “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 tackle such schools in his speech at last year’s Conservative Party Conference. He pledged to “shut down” schools, including Islamic madrassas, that are “filling children’s heads with poison and hearts with hate.”

“Let me be clear: there is nothing wrong with children learning about their faith, whether it’s at Madrassas, Sunday Schools or Jewish Yeshivas,” he said.

“But in some Madrassas we’ve got children being taught that they shouldn’t mix with people of other religions; being beaten; swallowing conspiracy theories about Jewish people.

“These children should be having their minds opened, their horizons broadened – not having their heads filled with poison and their hearts filled with hate.

“So I can announce this: If an institution is teaching children intensively, then whatever its religion, we will, like any other school, make it register so it can be inspected.

“And be in no doubt: if you are teaching intolerance, we will shut you down.”

Two months ago, inspectors shut down three unregistered schools in Birmingham after conditions were described as “unhygienic and filthy”. One was found to be using “anti-Semitic, homophobic and misogynistic material”.

School inspectors have so far found 15 “hidden” schools, operating full timetables, in the past year. A large number of them cater for the Muslim community.

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.”