Two years after the Trojan Horse scandal, children in Birmingham are still not being kept safe, Sir Michael Wilshaw has warned.
In a newly published letter, the Ofsted chief inspector says he has continuing concerns about the performance of the city's council and its ability to protect and ensure the safety of its children.
In 2014, Birmingham found itself at the centre of the so-called Trojan Horse controversy, which centred on an alleged move by a small group of hard-line Muslims to seize control of a small number of the city's schools.
The allegations sparked investigations by several agencies including the Department for Education and Ofsted.
Sir Michael's latest letter, addressed to Education Secretary Nicky Morgan, says it is two years since he wrote to her predecessor Michael Gove following inspections of 21 state schools in Birmingham, warning that the local council had failed in its duty to help schools to keep pupils safe from the potential risks of radicalisation and extremism.
"The schools that were placed in special measures as a result of Ofsted's findings have undergone changes of leadership and governance in the intervening period and are now generally improving," he says.
"Two of the schools at the centre of the so-called 'Trojan Horse' episode have been upgraded from inadequate to good in their recent re-inspections."
He adds he has continued make visits to the city, meeting with officials from schools, the council and police.
"As a result of these discussions, I am quite clear that, although many of these schools have improved and children are now much safer, the situation remains fragile." Sir Michael says.
"While the overwhelming majority of parents support the changes that have taken place over the past two years, there are a minority of people in the community who are still intent on destabilising these schools."
Sir Michael's letter also warns that Birmingham's children's services department has failed seven inspections in the last decade and that children in the city remain at risk.
"Despite the appointment of a succession of commissioners to the city, there has been little tangible improvement to the overall quality of child protection services," the chief inspector says.
"I have previously remarked that this long and shocking track record of inadequate provision represents a failure of corporate governance on a grand scale.
"I regret to inform you that I have seen nothing in the intervening period to alter this view.
"Birmingham's political leaders, in my opinion, have consistently shown themselves to be incapable of delivering the urgent and sustained change required to improve the safety and well-being of the city's vulnerable children."
Sir Michael adds that management of Birmingham's children's services has been handed over to an independent trust and urges Mrs Morgan to ensure that this trust is independent and "not influenced by those in the local authority who have demonstrated such incompetence over many years".
The letter goes on to list in detail a number of concerns, including those raised by inspectors that council staff are too slow in checking the whereabouts of children missing from education, and that there are not tough enough checks on youngsters whose parents choose to home education them.
Sir Michael concludes that some of the "serious shortcomings" he outlines are not confirmed to Birmingham City Council and that he is worried about the ability of a number of authorities to address risks in their area.
"In previous advice letters, I have highlighted the potential risks to those children who are being educated at home or in unregistered schools, as well as those attending some independent faith schools," he says.
"I have also identified issues around the ineffective tracking by some local authorities of pupils who are taken out of mainstream schools at points other than the usual transition dates.
"I have recently visited both Bradford and Luton to learn more about the level and quality of safeguarding in these areas."
He adds: "From my meetings with senior officials and elected councillors from both local authorities, it was clear that much more needs to be done to ensure that the possible risks to certain groups of children are fully understood and acted on."
Sir Michael calls on Mrs Morgan to agree for an Ofsted inspector to be assigned to any local authority where the government considers that children "are at a greater risk of radicalisation or their safety is being put in jeopardy by poor safeguarding practices".
Councillor Brigid Jones, cabinet member for children, families and schools, said: "Given that no-one from the political leadership has been interviewed in an Ofsted inspection since 2014, and that it has changed quite significantly since then, we found the comments in Sir Michael's letter to be a surprise. Our doors are always open should Ofsted wish to talk to us about their concerns directly.
"We will continue to do our utmost, working with partners and our commissioners, to ensure we have the right framework for social work in this city to become excellent. We have 800 dedicated social workers and a similarly committed wider team of professionals who support and work alongside them. They deserve recognition and credit for all they've done so far in the improvement journey - and all that they will do in the future."
She added: "In terms of the Chief Inspector's comments on schools in Birmingham, we note what is being said but contend they don't fully reflect the feedback that we receive from teachers, our education commissioner and the positive views of other government departments on our work on extremism."
A Department for Education spokeswoman said: "Although Birmingham City Council has made some improvements to the way it runs its children's services, we know this progress has not gone far enough, fast enough, and Sir Michael Wilshaw's letter reinforces that.
"That's why we have already announced we are working with the Council on the development of a voluntary Trust to make sure children and families in Birmingham receive the best possible care and support. On top of this, we are delivering a comprehensive package of reforms to radically improve child protection and the raise the status of social work."