Amanda Spielman, the chief of the Ofsted -- the Office for Standards in Education, Children's Services and Skills -- earlier this week released an annual report, warning that "a rising number of conservative religious schools were actively undermining British values and equality law".
She has been outspoken about her agenda to ensure all schools in the country follow equality laws and advocate British values.
She revealed that her clampdown against dubious institutions has not been received well among some hardliners, forcing one of Ofsted's regional offices even resorting to deploying extra security measures in the face of threats.
"I had an email, which was the most threatening one, which was along the lines of 'We know where you live and we can get you any time we want to'," she said.
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In one instance, Ofsted inspectors were so shocked by the aggressive behaviour of staff at one unregistered Islamic school in east London that they reported them to the police.
At another independent Muslim school, the headteacher telephoned parents during the Ofsted visit to tell them that inspectors were asking their children if they were gays.
Another 25 mixed faith schools will have to follow suit as a result of the ruling by judges in the appeal court.
Spielman revealed that in a handful of schools, inspectors had found cases of sexist and sectarian literature.
"If we let ourselves be intimidated out of discussing these issues, it's children who will suffer," she said, adding that she is determined to ensure that schoolchildren in the UK do not view British values as "meaningless".
The UK's Department for Education said it has changed the law and the requirements on schools "so that they have to actively promote the fundamental British values of democracy, the rule of law, individual liberty and the mutual respect and tolerance of those with different faiths and beliefs".