Last April, the Prime Minister ordered a review into the Muslim Brotherhood here in Britain. It's not before time.
Documents uncovered by US law enforcement agents in 2004 showed that the Brotherhood had planned to launch a stealthy "kind of grand Jihad in eliminating and destroying the Western civilisation from within… so that… God's religion is made victorious over all other religions."
One document earmarked 29 Islamic organisations to help the Brotherhood "expand the observant Muslim base" as a first step to this fantastical goal, including funding Hamas.
The US authorities broke up the plot. There is, however, compelling evidence that the American blueprint was partially replicated here. Unlike the Americans, the British authorities seem just to have shrugged their shoulders.
Now Mr Cameron wants to know what "wider influence" the Brotherhood has had "on UK society" and the history of its involvement here.
Tower Hamlets has begun to look more like Riyadh than the secular Bangladeshi capital, Dacca
The Brotherhood as an organisation was founded in Egypt in 1928 with the slogan: "The Quran is our constitution. The Prophet is our leader. Jihad is our way. Death for the sake of Allah is our greatest wish."
Since then, the organisation has morphed into a wider, fragmented movement across much of the globe, including Britain.
As preposterous as the Brotherhood's "grand jihad" might seem, it has been echoed by the movement's ageing, de facto spiritual leader, Sheikh Yusuf Qaradawi. Europe's destiny, he says, is to be conquered by Islam, "not by the sword" but by peaceful proselytising.
The more pertinent question is: what has been the impact of the Brotherhood movement's attempt to "expand the observant Muslim base" here in Britain to mainstream efforts to create a meaningful common life between Muslims and non-Muslims?
The immediate trigger for the Cameron review was the escape to London of Brotherhood leaders from Cairo after the military overthrew Egypt's first democratically elected Brotherhood government. The Egyptian government has said the Brotherhood there have links to gunmen who have shot policemen.
The Brotherhood emphatically denies this, insisting they are committed to democracy.
For many, the Brotherhood remains a conundrum: how can their commitment be reconciled with their version of Islam which makes no distinction between Islam as a spiritual faith and a political ideology?
Some say that, because Tunisia's elected Brotherhood government peacefully ceded power to a secular caretaker government, democratic shoots can grow from the Arab Spring.
Sceptics counter that the Brotherhood plays a long game and that their leading ideologues still believe the world is ultimately destined to be reorganised around Islam, with the Quran as the sole basis for government and law.
The Cameron review will also examine the south Asian cousin of the Brotherhood Movement - the Islamist Jamaat e Islami, which enjoys greater support among British Muslims than its Arab equivalent. Though untainted by the 2004 plot, the Jamaat was created by an Indian theologian Sayyid Abul A'la Maududi, who wrote that Muslims were obliged to establish the sovereignty of God on earth.
It is generally accepted that Maududists founded the UK Islamic Mission (UKIM) in 1962. Today the Oldham-based UKIM still says it is a social and political "ideological movement" which exists to "mould the entire human life according to Allah's revealed Guidance."
However, UKIM's claims that all this is aimed at promoting "harmony in society and the entire world" were undermined in 2007 when a Channel 4 investigation showed the UKIM hosting extremist preachers. One was recorded as urging Muslims to "help us win the fight against the Kuffar [unbelievers]… in every department of life". Another said Muslims in Britain had to "live like a state within a state" and continue to preach "until you become such a force that the people they just submit to you, hands up, until you become strong enough to take over".
Although Jamaat e Islami-inspired organisations control just three per cent of mosques, a government-published report has said that the "JI helped to create and subsequently dominate the leadership" of the organisation that describes itself as the "national representative" of Britain's very diverse Muslims - the Muslim Council of Britain.
In parts of Britain, there are signs of a growing religious conservatism among the rapidly growing Muslim population, who appear increasingly resistant to assimilate into the majority culture, as most other migrants have over time.
Neither the MCB nor UKIM seems concerned that treating more and more citizens on the basis of their faith rather than common citizenship might be a significant obstacle to creating a society which is genuinely cohesive.
It is true that, on some cultural differences like female genital mutilation, sexual grooming, and forced marriage, the MCB has spoken out. But they have tended to follow the debate - not lead it.
Their boycott of Holocaust Memorial Day also burned bridges built up with the Jewish community over decades.
In 2007, the MCB tried to inject a conservative version of Islam into state education with a list of multiple requirements. Some were introduced in Birmingham state schools, recently adjudged by Ofsted to have so narrowed the curriculum and exposure of their Muslim pupils to other cultures and faiths that they were being badly prepared for life in modern Britain.
Under the influence of the Jamaat's Bangladeshi branch, Tower Hamlets, the London borough which is home to 80,000 Britons of Bangladeshi heritage, has changed profoundly over the last three decades.
With ever more women covered with full-faced veils, Tower Hamlets has begun to look more like the Saudi capital, Riyadh, than Dacca, the relatively secular capital of Bangladesh.
The Jamaat dominate the influential East London Mosque, and while it has contributed to useful social welfare programmes, judging by some speakers to whom the mosque still gives platforms, its definition of extremism still conflicts with the one used by the government's Prevent strategy, challenging the ideological narrative of terrorism.
The Brotherhood movement's Arab wing in the UK controls even fewer mosques than the Jamaat. Although both have promoted Islamist ideology, the principal focus of the Arab Brothers has been Hamas.
They have established a powerful and active Hamas support network, which has its origins with the arrival in Manchester of radicalised Palestinian students in 1979.
Hamas supremo Khaled Mishaal says they were the "first pillars of Hamas", building up support "on the outside" for its official launch in 1987.
Student branches were also established in Europe, the Gulf and America. Called the Islamic Association of Palestinian Youth, the then functioning British branch was among the most active, and like the others, part of what a seminal Hamas memorandum has described as the "Hamas Project".
Charities were listed as integral to this "Jihadi [struggle] project" to replace Israel with an Islamic state, by providing food, medical care and education to generate loyalty and support for "the [Hamas] Movement" to keep "the flame of Jihad alight.."
Funds for Hamas's welfare network were sent from charities in Britain and America established and run by members of the International Muslim Brotherhood's "Palestine Section" whose job was to oversee the "Hamas Project."
From a north London flat, they also ran a monthly glossy magazine, Fillisteen al Muslimah [Muslim Palestine] which published heroic photographs of suicide-belted martyrs.
Within days of 9/11, the British authorities were eyeing up the Brothers as "credible" non-violent extremist partners against the violent extremism of Al Qaeda - a role the Brothers relished. Playing gatekeeper to Britain's Muslims has been a key Brotherhood aim.
An early approach came from MI5 when two officers gingerly knocked on the door at Fillisteen al Muslimah to ask if they might possibly come in for a chat.
Struggling to suppress their guffaws, through the door the Hamasniks told them to come back after they had made an appointment.
The Blair government soon realised that relying on the Brothers as interlocutors in the battle against violent extremism was double-edged.
While the Brothers condemned terrorist outrages like 7/7 here at home, as David Goodhart, director of the Demos think tank, says, their own beliefs were often a "non-violent version" of the violent version: "religiously-inspired hostility to foreign policy, ambivalence about Western liberalism, and a paranoid belief in state sanctioned Islamophobia."
The false but inflammatory grievance narrative that Britain has been fighting a global war directed against Islam has also been reinforced by "Islam Channel", the most popular Islamic satellite TV station in Britain today.
Between programmes, Muslims are reminded again and again that the channel gives a "voice to the voiceless… a voice to the oppressed."
By helping to promote an extreme world view that the West has been engaged in a conspiracy against Muslims, the Brotherhood has helped keep young British Muslims angry - and somewhat separate.
Some Hamas fugitives have been given sanctuary here from Israel only to repay Britain by using it as a base to work against British foreign policy on Iraq, Afghanistan, Iran and especially the Israel-Palestine conflict with regular trips to actively assist Hamas leaders undermine the - admittedly vanishing - prospects of a two-state solution.
So far, it is hard to see what material contribution the Brotherhood movement has made to creating the harmonious and cohesive society they say they want. They do, of course. But on what terms?