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."