The head of the UK's education watchdog is calling on head teachers not to be afraid of causing offence and tackle religious extremists who use schools to indoctrinate children.

In a strongly worded criticism of religious communities wanting to limit education opportunities, Amanda Spielman will outline in a speech on Thursday (1 February) her concern that some schools are being used by to espouse, in the worst cases, "extremist ideology".

"Ofsted inspectors are increasingly brought into contact with those who want to actively pervert the purpose of education," she will say.

"Under the pretext of religious belief, they use education institutions, legal and illegal, to narrow young people's horizons, to isolate and segregate, and in the worst cases to indoctrinate impressionable minds with extremist ideology."

Spielman is backing Neena Lall, the head of the UK's top state primary school, St Stephen's in Newham, east London, who banned younger pupils from Ramadan fasting in school hours and girls under eight from wearing the hijab in class.

Lall has been vilified by a group of parents and leaders of the predominantly Pakistani and Bangladeshi community who circulated a video of her comparing her to Hitler.

But Spielman will tell the Church of England schools' conference: "Freedom of belief in the private sphere is paramount, but in our schools it is our responsibility to tackle those who actively undermine fundamental British values or equalities law," the Times reported.

Spielman took legal action against Al-Hijrah, a state-funded faith school in Birmingham, to stop it segregating girls and boys on religious grounds and another 25 mixed-faith schools will follow suit after a court ruling. She says head teachers must not be afraid to cause offence.

"Rather than adopting a passive liberalism, that says 'anything goes' for fear of causing offence, school leaders should be promoting a muscular liberalism.

"It means not assuming that the most conservative voices in a particular faith speak for everyone — imagine if people thought the Christian Institute were the sole voice of Anglicanism.

"And it means schools must not be afraid to call out practices, whatever their justification, that limit young people's experiences and learning," the Times reported.