In one case, a teenage student now in their 20s -who studied at private schools in ultra-orthodox communities - had the level of education of an eight year old.

It is understood both the Department for Education (DfE) and Ofsted, the schools watchdog, are looking for them to shut them. However, it emerged some of the illegal schools are registered as charities, which grants them tax breaks.

These private schools are located in the small Charedi community - which is made up by a number of strictly Orthodox Jewish traditions.

Some parents in this community demand a narrow education for boys, one that is largely focused on religious education and delivered in the Yiddish language. Girls are offered a relatively mainstream education, however.

There are concerns that many Charedi schools aren't offer a broad enough curriculum, therefore preventing boys from leaving the community.

One ex-student of illegal Charedi schools, now in his 20s and outside the community, told Newsnight: "I'm starting to study for my GCSEs. I'm maybe like an eight-year-old, nine-year-old. That's my level of education." The report follows concerns from Sir Michael Wilshaw, the chief of Ofsted, who announced late last year that illegal schools face criminal prosecution.