Some of Birmingham's schools have seen an "organised campaign" to spread an intolerant version of Islam via the use of governors, Ofsted has found.
In a report which goes to the heart of the ongoing row between Michael Gove, Theresa May and David Cameron, Ofsted chief Michael Wilshaw found five schools had failed to protect their pupils from extremism and had to be put in special measures.
Wilshaw delivered the statement without taking questions, but this remains a difficult time for Ofsted, which is facing questions itself about why it failed to report on the alleged failings during its last report.
"A culture of fear and intimidation has taken grip" in Birmingham schools, Wilshaw said.
"Some of our findings are deeply worrying and in some ways, quite shocking.
"Governors are exerting far more influence than is appropriate or acceptable."
He found headteachers were being undermined at the hand of the new governors and that curriculums were being narrowed to reflect their personal views.
"Some teachers reported that they were treated unfairly because of their gender or religious beliefs," he said.
Wilshaw singled out Birmingham City Council for criticism, saying its "serious failure" to support schools had left children vulnerable to extremism and accused it of a "lack of urgency".
The report does not find examples of extremism being spread, but it found institutions had failed to protect children and had not encouraged them to "develop tolerant attitudes towards other faiths".
A sixth school was labelled inadequate for its poor educational standards and another 12 were told to make improvements.Ofsted will be able to conduct snap inspections of any school with no warning, under new plans being drawn up by the prime minister ahead of what is expected to be a damning report into extremism in Birmingham schools.
Cameron brings in no-notice Ofsted inspections
The move comes as David Cameron tries to get on top of the row about supposed Trojan Horse extremism in schools.
Education Funding Agency (EFA) reports suggest that notice periods were used by schools in Birmingham to put on hastily arranged shows of cultural inclusivity.
The EFA report into Oldknow school will say that staff informed inspectors that they had been instructed to add Christianity to the learning schedule because of the visit.
"We were told by two staff members that the assembly [on Easter and Christianity] had also been put on especially for our benefit," the report will find.
A timetabled literacy lesson was also switched to an RE lesson on Christianity, supposedly also for inspectors' benefit.
The previous Ofsted inspections of the so-called Trojan Horse schools in 2012 - which found them good or outstanding - gave one to two days' notice, but the current inspection gave just 30 minutes notice or less.
Under current rules, only schools already rated inadequate for behaviour routinely face unannounced inspections, although the chief inspector has the power to order no-notice inspections if there is justification for doing so. This is usually due to serious child safeguarding concerns, as in Birmingham.
But in advance of today's reports, education secretary Michael Gove has asked Wilshaw to report back on the practicalities of allowing the inspection of any school at no notice.
Not every inspection would be no-notice, but every school would be aware that they could face a no-notice inspection at any time.
Cameron also chaired a meeting of the government's extremism taskforce today to discuss the findings of the Ofsted review.
In a frantic day of meetings ahead of a trip to Sweden the prime minster also held talks with Nick Clegg, George Osborne, Michael Gove, Theresa May, Eric Pickles, Baroness Warsi and David Laws.
Ofsted will be told to maintain a regular presence in Birmingham schools, with inspection reports going to the prime minister in addition to the education secretary.
"Protecting our children is one of the first duties of government and that is why the issue of alleged Islamist extremism in Birmingham schools demands a robust response," the prime minister said.
Labour criticised the prime minister's response as "weak".
Shadow education secretary Tristram Hunt said: "This is an entirely weak and inadequate response by David Cameron to the gravity of the situation in Birmingham. What we're seeing in Birmingham is part of the systematic failings in the current school system.
"Cameron’s schools policy has delivered a vacuum in the local oversight of our schools, leaving children exposed to falling standards and vulnerable to risks posed by extremists.
"By refusing to address the inadequate checks in the schools system, Cameron reveals he is happy with the status quo."
Gove will respond to questions about the report when it is published this afternoon, in what looks set to be a humiliating Commons performance.
Over the weekend he wrote a letter of apology to the prime minister and Charles Farr, director of the Office for Security and Counter-Terrorism, after he and Theresa May became embroiled in a tit-for-tat briefing war over who was responsible for the Trojan Horse plot.
May paid an even higher price and lost her trusted special adviser, Fiona Cunningham.
She will be questioned about the row by the home affairs committee, which may also call her former special adviser to give evidence.
"I have written to the home secretary for a full explanation of what has happened," committee chair Keith Vaz said.
"The committee will in due course question her about these matters. There is a strong case to hear from Ms Cunningham herself as to why she has resigned."