Sir Michael Wilshaw said the pupils' "physical and educational welfare is at serious risk" following a series of emergency inspections.
He said all the schools focused too heavily on Islamic teachings.
Education Secretary Nicky Morgan says the schools will be closed down if changes are not made quickly.
She said: "We asked Ofsted to carry out these independent school inspections and the findings are very concerning. While there is no suggestion of a co-ordinated plot, it is clear that these schools are failing children and this is unacceptable.
"All schools must prepare children for life in modern Britain."
At one school, inspectors found pupils did not know the difference between sharia and British law.
And they said the curriculum at Mazahirul Uloom School in Tower Hamlets "focused solely" on Islamic themes. [BBC] Read more
