Pupils at six small Muslim private schools in east London are at risk of extremist views and radicalisation, says Ofsted's chief inspector.
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.
One of the schools called Ofsted "unprofessional", while another said its findings did not reflect reality.
Education Secretary Nicky Morgan says the schools will be closed down if changes are not made quickly.
"We asked Ofsted to carry out these independent school inspections and the findings are very concerning," she said.
"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.
In a letter to Ms Morgan, Sir Michael said he was "extremely concerned about the large number of failings" in each of the six schools and was "not convinced" current managers were capable of making necessary improvements.
"I believe that, in all six schools, pupils' physical and educational welfare is at serious risk," he wrote.
"Given the evidence gathered from these inspections, particularly in relation to the narrowness of the curriculum, I am concerned that pupils in these schools may be vulnerable to extremist influences and radicalisation."
By Caroline Wyatt, BBC religious affairs correspondent
The recent downgrading of several Muslim schools suggests a growing nervousness about Islam in the UK, and what they are teaching or allowing on their premises.
Other faith schools have been inspected, with some found not to be teaching enough about other faiths and cultures.
The inspections also suggest wider social concerns about the make-up and cohesiveness of British society after years of immigration, and over whether faith schools, in particular, prepare pupils to play their part as full UK citizens.
The debate over "British values" came to the fore in the wake of the "Trojan horse" affairs, and the realization that hundreds of British Muslim men - and some women - had become radicalised enough to join extremists in Iraq and Syria.
The government has stressed "fundamental British values" must be taught and encouraged in schools.
Secular and humanist campaigners have welcomed an increase in inspections, saying that for too long the UK has allowed religious communities to "enforce their own values and traditions" on children.
Mazahirul Uloom, a small secondary boys' school that professes to teach the National Curriculum and Islamic Sciences, faces the most criticism.
Inspectors said too much of the curriculum "focuses solely on Islamic themes" and judged it inadequate.
They said pupils believed it was wrong to learn about other religions, were not taught art, music or drama and had a "narrow view" of women in society.
Some students told inspectors: "Women stay at home and clean and look after the children. They cook and pray and wait for us to come back from school with homework."
But the school said that during a two-day inspection in October, Ofsted asked pupils "vaguely worded questions which produced vague responses".
"To make sweeping generalisations on the basis of their response is utterly unprofessional," it said in a statement.
"As some of these students had just begun their schooling, it is preposterous to suggest their views somehow reflect the school's curriculum."
Suggestions that children were not protected from extremist views were "completely unfounded", it said, adding that Ofsted's findings regarding the role of women did not "reflect the school's attitude".
The school said it was "natural" for an Islamic school to have a "primary ethos" based on Islam, but that did not mean it taught children that other faiths and traditions were "antithetical to Islamic teachings".
An emergency inspection of the 185-pupil boys' secondary Jamiatul Ummah found good opportunities to study and practise the Islamic faith but pupils were not provided with a broad and balanced curriculum.
It was previously judged outstanding but has been downgraded to inadequate.
The report said: "The narrowness of the curriculum means that students' spiritual, moral, social and cultural education, in particular their understanding of the fundamental British values of democracy, the rule of law, individual liberty and mutual respect and tolerance, is underdeveloped."
Students also lacked opportunities to learn about music and art, or to be creative in PE.
A statement from Jamiatul Ummah said it was disappointed with the findings but that it would work hard to address concerns.
"Ofsted has given disproportionate emphasis to certain issues which do not reflect the real characteristics of the school and has not portrayed accurately the school or given appropriate weight to the varied educational experiences, including National Curriculum," it said.
It stressed that it had 100% of pupils getting five good GCSEs in English and maths last year.
The findings of the other four Ofsted inspections included:
- Ebrahim Academy - (secondary) pupils not prepared for modern British life and curriculum said to be too narrow;
- London East Academy - (secondary) rated inadequate. Curriculum is not broad and balanced, and students have insufficient understanding of how other people live in Britain and abroad. Also school's work to keep children safe is inadequate. Most of the school library books are in Arabic;
- Al Mizan - (primary) rated inadequate. Work in religious studies books shows many pupils have learned only about Islam. Systems for keeping children safe are weak - public have open access to school;
- East London Islamic School - (primary) rated inadequate. The majority of lessons focus on Islamic or Arabic studies. All pupils learn to recite religious texts by memory and repetition. Teachers adopt a similar teaching style in other lessons.
The publication comes a day after details leaked of a report on another school, Sir John Cass Foundation and Redcoat, in Stepney, east London.
This is the only state and non-Muslim faith school in the group to be inspected as part of the same set of seven snap inspections and failed for not safeguarding and monitoring pupils adequately.
The report highlighted the segregation of boys and girls in indoor and outdoor play areas and the risk of extremism.
The six private schools are all in Tower Hamlets, where the council said it had no jurisdiction over teaching and standards at independent faith schools and that its powers were limited to offering training and advice to schools.
"We have repeatedly offered this assistance to independent schools locally but we cannot compel them to accept this help," it said.
"We can - and we do - intervene when individual safeguarding issues are raised."
Independent schools, academies and free schools already have to adhere to the Independent School Standards, which demand that schools encourage pupils to "respect" British values.