Some religious schools are being used to 'indoctrinate impressionable minds with extremist ideology' under the guise of religious belief, the head of Ofsted is warning.

Extremists use schools to 'pervert' the purpose of education, Amanda Spielman, the head of the schools' inspectorate body will say, calling on head teachers to 'tackle those who actively undermine fundamental British values'.

In a speech to the Church of England's education conference on Thursday she will warn schools that they should not assume the 'most conservative voices' within a faith represent everyone. While insisting that faith schools generally are good at promoting tolerance, she will distinguish between respecting religion and 'privileging all belief above criticism'.

'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.

'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.

'That doesn't just mean Ofsted, but everyone involved in education.

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

'Occasionally that will mean taking uncomfortable decisions or having tough conversations. 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 in school.

'In that regard schools must not, in their entirely correct goal of promoting tolerance, shy away from challenging fundamentalist practice where it appears in their schools or communities.

'Similarly schools must not allow pressure from certain elements of school communities to dictate school policy, nor should we allow vocal parental minorities to pressure other parents and children to act or dress against their wishes. Giving way to the loudest voices is the opposite of tolerance.'

Spielman also threw her weigh behind Neena Lall, the head of St Stephen's primary school in east London, who tried to ban girls under the age of eight from wearing hijabs and prevent younger children from taking part in the Ramadan fast during school time.

Ms Lall was subsequently compared to Adolf Hitler in a video circulated among parents and the school was forced to reverse its decision.

Ofsted will 'always back heads who take decisions in their interests of their pupils', Spielman will say, adding that 'school leaders must have the right to set school uniform policies in a way that they see fit, in order to promote cohesion'.