Inspectors have been sent by the Department for Education into Hackney, north-east London as part of a drive to register strictly Orthodox yeshivot.
According to the law, children aged 16 and under must be taught in institutions officially recognised by the state.
But the authorities have voiced concern that as many as 1,000 Charedi boys in London aged from 13 to 16 are receiving education in unregistered yeshivot.
A DfE spokesman confirmed that officials from the government’s inspection service “visited a number of addresses in Hackney to check whether a school was being operated on the premises. If a school is providing full-time education for five or more pupils of compulsory school age, it should be registered as an independent school”.
The DfE declined to give details of which institutions had been visited, saying that if the visit were not for the purposes of a normal school inspection, names would not be published.
Before a school is accepted for registration, it would have to pass an Ofsted inspection.