The sadistic beheading of the American journalist James Foley by an ISIL killer apparently from Britain, just a few days ago, is the first such killing of an American by a jihadist with a British passport.

But it is the second ritual beheading carried out by British Islamists (the head of a British soldier, Drummer Lee Rigby, was hacked from his body by two of them in Woolwich, London last year in full public view).

Jihadists from Britain are at the forefront of the most violent extremism seen in modern times and many will properly be puzzled by how such people be citizens of a civilised country like Britain and why we seem powerless to prevent them from behaving like this.

On 8 October last year Andrew Parker, head of MI5, Britain’s security service, said there were ‘several thousand Islamist extremists’ in the UK. He also said that the UK has ‘one of the most developed and effective set of counter-terrorist capabilities and arrangements in the world’. Adding ‘for the future there is good reason to be concerned about Syria. A growing proportion of our casework now has some link to Syria, mostly concerning individuals from the UK who have travelled there to fight or who aspire to do so. Al Nusrah and other extremist Sunni groups there aligned with Al Qaeda to attack western countries’.

If we knew all this last autumn, and if our capabilities and arrangements are so superb, why have we not only failed to eliminate the jihadist danger but actually seen it increase? Today about 500 young Muslims from Britain have travelled to Syria, turning jihad into a gap-year activity.

One answer is that instead of quizzing Parker (and his colleagues from MI6 and GCHQ) as to what should be done about several thousand extremists in Britain, Britain’s intelligence community was stunned by a barrage of criticism from civil liberties groups and the libertarians in the Tory and LibDem parties, a bizarre coalition, which was frequently joined by prominent ‘human rights’ lawyers.

Already under attack from this lobby thanks to the appalling activities of Edward Snowden, and of Julian Assange before him, our intelligence chiefs found themselves having to justify their work on our behalf instead of being able to request more resources and firmer policies to make carrying it out easier for them.

Just a few days ago another jihadist from London, known previously only as a rapper, whose music was broadcast on the BBC, was seen in ‘the Islamic State’ proudly holding the severed head of a soldier under the caption ‘Chillin’ with my homie of what’s left of him’.

Another Brit, Reyaad Khan, 20, from Cardiff boasted online of his ‘martyrdom ops’, ‘planning “fireworks” ‘ and ‘executing many prisoners’. Abdul Amin, an engineering student from Aberdeen texted that joining ISIL was one of the ‘happiest moments of his life’. There are many other like these: the list is very long. Foreign Secretary Philip Hammond has admitted that ‘significant numbers’ of Britons are involved in the commission of atrocities’.

It is now obvious to everyone that almost ten years after the London bombings, Britain has a serious and growing problem when it comes to young British Muslims becoming radicalised and turning to terror. What now needs to be reflected upon is why this should be the case – and what our policy makers must do about it.

Part of the problem is that many Muslims in Britain come from parts of the world like Pakistan, Bangladesh, the Horn of Africa, where political violence is endemic. Yet the biggest single trigger of jihadism here has been our adherence to ‘multiculturalism’ which has meant that we have for far too long allowed vile Islamist ideologies to be propagated under the cover of ‘free speech’ or ‘religious freedom’.

Islamists in Britain have been able shamefully to exploit our proud tradition of freedom and staying out of religious disputes, seen as questions of personal faith. We have closed our eyes to the reality that to fight for ISIL and to slaughter and maim on its behalf is a political act, not a religious one.

Of course, the so-called preachers like Anjem Choudary repeatedly claim that Islam justifies the butchery and genocide. However because Sunni Muslims have no recognised religious hierarchy anyone can call themselves a ‘preacher’ and go on the campus lecture circuit, to mosques or other places young Muslims congregate. They are even able to gain funding from an Islamic ‘charity’ which arranges their tours and funds their expenses.

What’s more, declaring for ISIL is not like joining a mainstream political party or a college debating society. Those who fight for it explicitly want the overthrow of parliamentary, liberal democracy, the establishment of a world caliphate – the black flag of ISIL to fly over Buckingham Palace as one young jihadist put it a few days ago. For the jihadist, ‘we’ means the jihadists and ‘you’ means us, the Brits. Islamists hate Britain and despise its values. They say so. Their black flag has nothing to do with Islam, but everything to do with political power and domination.

Since the Cameron-Clegg coalition came to power in 2010, specific measures (such as control orders) have been abolished and replaced with much milder ones (Mrs May, the home secretary, said this was to ‘restore civil liberties’) and the Communities minister, Eric Pickles, has made it clear he is bored by counter-radicalisation measures regarded as wasteful (which some often were) and which smack of Blairism. Whilst Cameron himself (for example in his 2011 Munich speech) publicly railed against extremism, his government quickly lost its appetite to act against it.

Even so, four years of counter-radicalisation measures, even weak ones, have not helped us win the battle against murderous Islamism. It is plain, therefore, that the time has now come to go much further than ever before in stamping jihadism out of British life.

For one thing we need to scrap the concept of multiculturalism, of letting different cultures co-exist inside Britain as if they were equal to our own free, western values. Sharia courts in the UK should be immediately dismantled. We need to pull ‘faith’ out of education. ‘Faith schools’ (another Blair obsession) have been cynically used by Islamists to peddle their own political views, promote Sharia law, and anti-Semitic and anti-women attitudes (as was seen recently in Birmingham).

This is truly about politics, power and political systems, not religion. One ISIL recruit from East London told the BBC ‘the caliphate is something that is in the heart of every single practising Muslim. There is not a single country in the world, whether it’s Saudi or Pakistan that implements Islam fully. So now we’ve got this caliphate, people are going to flock there and leave the west and live their [sic] peacefully under the Sharia’. Here we have it: this brainwashing (which extends to the belief that living in the Islamic State is ‘peaceful’) didn’t just happen by itself; it was done to this man and thousands like him by odious demagogues.

It should be made clear to everyone that if you live in Britain, you live by British rules. If you don’t like them, you are free to go to a country that has the rules you do like. Instead of preventing radicalism by bringing back ineffective control orders, institutions who allow extremists onto their premises to brainwash young Muslims should be prosecuted and if necessary closed down. Foreign ‘preachers’ should be kept out of the country.

Finally, rather than try to prevent jihadists from Britain going off to Syria by confiscating their passports, we should let them travel and take their passports off them only when they’ve left Britain. They should never be allowed to return. The sadistic murders and mass killings in which they revel show that whatever nation they belong to, it is not Britain.

Professor Anthony Glees MA M Phil D Phil (Oxon) is the Director of the Centre for Security and Intelligence Studies (BUCSIS) at The University of Buckingham