About 5,000 UK children are being taught the Saudi national curriculum
What is extremism - how do you define it?
The Education Secretary Michael Gove says: "You know it when you see it."
That does not seem to be the case for some part-time Saudi schools in Britain.
As Panorama has discovered, a network of more than 40 weekend Saudi schools have been offering to teach the Saudi national curriculum to Muslim children in the UK and some of it is very extreme indeed.
Text books teach school children about the correct way to chop off the hands and feet of thieves - a hand for the first offence, a foot for the second.
It tasks pupils to spell out the "reprehensible qualities" of Jews. It explains that the penalty for sodomy is execution - possibly by being thrown off a cliff - and that Zionists are plotting to take over the world for Jews.
This is a reference to a late 19th century "plot" by Russian anti-Semites who claimed the elders of Zion had written a series of secret "Protocols" to achieve global domination of the economy and media.
These "Protocols" were exposed as a hoax 89-years-ago.
But for 15-year-old Muslims studying the Saudi national curriculum here in Britain in the hope of getting into a Saudi university, the plot continues to the present day. There are "many proofs" of the Protocols' "veracity", the grade 10 text book says.
BBC Newsnight exposed similar anti-Semitic sentiments in Saudi text books at a Saudi school in London in 2007. The Saudi government's promise of a "comprehensive" review of all offensive material in their curriculum does not seem to have been comprehensive enough.
These Saudi schools have been going for about 30 years.
The Saudis have got away with this for so long because part-time schools do not have to register with the Department for Education (DfE), so they do not get inspected. That may now change.
Mr Gove told me the school inspectorate Ofsted will shortly report to him about how "part-time provision is better registered and better inspected".
Easier said than done.
We are determined to ensure that those who have access to public money and those who are shaping young minds, do not peddle an extremist agenda.
Michael Gove, Education Secretary
Thousands of children from all faiths attend other perfectly respectable part-time schools at the weekend. And these classes are not just held in church halls, mosques, synagogues, and temples. Some are in the backrooms of private houses.
What about full-time faith schools of which there are around 25,000 in England and Wales? Do they all recognise extremism when they see it? Assuredly most do. But not all. And this is where the spotlight again falls mainly on Muslim faith schools.
There are not as many Muslim faith schools per head of population as there are Christian (most of which are now 'faith' in name only) and Jewish.
But Muslim parents are turning to faith schools, mostly private, in increasing numbers in order to preserve the Islamic ethos of their offspring and to shield them from western influence.
In our comparison of more than 100 websites of schools of all faiths, it was the anti- western tone of some Muslim schools that was most striking.
Let us be clear. No-one is talking here about teaching terrorism.
The real concern is about a school ethos that reinforces separatism - either by living in separate enclaves or by not buying into the core values of a liberal democracy - tolerance, equality between men and women, and respect for man made laws.
The Muslim population of Britain has been rising rapidly and research by the economics department at Bristol University shows that Muslim children are the most segregated in Britain.
Faith schools are growing in popularity. Which way will Muslims be pulled - towards or away from the mainstream?
The new government, much like the last one, sees integration as the long term solution to radicalisation because it has recognised that a sense of Muslim isolationism from wider UK society can increase vulnerability to radicalisation by charismatic preachers.
That is why the entire school inspection system is now being reviewed by the Education Secretary.
Many Muslim faith schools clearly do manage to preserve their Islamic identity whilst embracing the pluralism of a liberal democracy.
Al Furqan school in Birmingham
I was struck how the Al Furqan school in Birmingham displayed symbols of all the faiths, including Buddhism and Hinduism, shunned by many Conservative Muslims as normalising polytheism - the worship of Gods other than God.
For me the symbolism seemed clear: Al Furqan was genuinely striving to connect to the mainstream.
Just as clearly, however, other Muslim schools we encountered seemed in varying degrees to want to stay separate, leading separate lives in separate enclaves.
Will these steer more Muslim faiths schools in the general direction of schools like Al Furqan?
The report finds that the education system is currently "not equipped" to prevent "extremist influences" in faith schools.
It includes Ofsted whose often glowing reports about a school's promotion of "spiritual, moral and cultural" values seem at variance with the school's links to fundamentalist preachers, Islamic institutions, mosques and websites.
One of the report's authors John Bald, a former Ofsted Lead Inspector, says Ofsted was never designed to sift and weigh evidence of this kind.
Policy Exchange also say the government should set up a dedicated Due Diligence Unit based at the DfE to vet new applicants who want to establish new academies and schools independent of government under Mr Gove's Free Schools plan.
I am told that currently due diligence consists of not much more than a "google search...of Companies House and Linkedin".
Mr Gove is already adopting some of these recommendations.
He told me he is not going to "attempt to police what people believe" but is "determined to ensure that those who have access to public money and those who are shaping young minds, do not peddle an extremist agenda".
Applications from "any organisation or individual who we fear might be associated with extremism" will be rejected, Gove explained.
As in other spheres, the attempt to shape a new British identity from childhood - one that is both British and Muslim at ease with their common citizenship - seems to have entered a new phase.
Panorama: British Schools, Islamic Rules, BBC One, Monday, 22 November at 2030GMT and then available in the UK on the BBC iPlayer.