Many more children are hidden from authorities in unregistered schools across the country than previously thought, Ofsted chief inspector Sir Michael Wilshaw has warned.
Inspectors have identified 100 suspected unregistered schools across the country since a team was set up in January to investigate the problem.
Last month, inspectors issued seven warning notices to suspected illegal schools in London, Birmingham, Luton, Wolverhampton and Stoke-on-Trent, where they found around 350 children on the premises.
In a letter to Education Secretary Nicky Morgan, he said inspectors had been “deeply alarmed” by some of the issues they have found.
He told her: “The evidence that they have gathered so far during this short period firmly reinforces my belief that there are many more children hidden away from the view of the authorities in unregistered schools across the country than previously thought.”
Sir Michael added: “What we have found so far is likely to represent only a small proportion of the illegal schools operating across the country. Inspectors are hearing about suspected new cases every week.
“I therefore remain extremely concerned about the number of children and young people attending these schools who may be at significant risk of harm and indoctrination.”
Ms Morgan asked Sir Michael last year to set up a new Ofsted taskforce to investigate suspected illegal schools, and pursue those responsible for running them.
Inspections uncovered “serious fire hazards, including obstructed exits and inaccessible fire escapes”, as well as schools with unsafe and unhygienic premises.
One case involved the discovery of “chemicals and chemistry equipment in an unlocked food cupboard in a room where children ate their lunch”, Sir Michael said.
Staff and volunteers were also found to have not been properly checked or cleared to work with children.
This comes after Hackney Council said it is “trying to tackle” the issue of unregistered Orthodox Jewish schools in Stamford Hilll.
They said this after a 200-student school defiantly stayed open despite a government closure order.
The Local Authority, which has legal responsibility for safe-guarding, was reacting to a story in The Independent, accusing it of “destroying evidence” about the extent of the problem.
A spokeswoman denied there was a “cover-up” but could not produce any notes of the meeting at which council and Department for Education officials discussed the problem, as referenced in the information provided by a later Freedom of Information request.
Rabbi Avroham Pinter, a spokesman for the Stamford Hill community, said: “The school has said it is closed, I have no information to the contrary. I understand the children are getting home schooling from the parents.”
In January, the government announced “an escalation of Ofsted investigations into unregistered schools, with additional inspectors dedicated to rooting them out, a new tougher approach to prosecuting them and a call to local authorities to help identify any setting of concern”.
Figures show that, every year, more than 450 Orthodox boys are in registered schools at the age of four, but this drops to about 50 boys by the age of 13. Most Orthodox girls continue in state education until they are 16 years old.