Thousands of children are being taught in unregistered schools across England, many more than previously thought, Ofsted's chief inspector has said.
Sir Michael Wilshaw said a crackdown had found more than 100 suspected illegal schools - half of which were faith-based, Ofsted said.
Roughly a third of them were Islamic and a sixth either Christian or Jewish. Seven warning notices have been issued to schools in London, Wolverhampton, Birmingham, Luton and Staffordshire.
Any school offering 20 hours of lessons a week must be registered.
Unregistered schools are those that operate outside the supervision of the Department for Education, local authorities or Ofsted inspections. They are often run by faith groups and there are concerns about the safety of pupils in their charge.
In a letter to Education Secretary Nicky Morgan, Sir Michael said his team of seven experienced inspectors, working closely with DfE officials, had identified more than 100 suspected unregistered schools across the country since January.
