LONDON — Back in the 1970s a political idea became very fashionable in Western Europe, and especially in Britain. This was the concept of multiculturalism — the belief that different immigrant and ethnic groupings, who were then pouring into the region, should be left to their own devices and allowed, even encouraged, to develop their own cultures.

For example, Britain, it was proudly declared, was to become a multicultural society in which all the different faiths and customs would be fully respected, in the hope that in due course they would all grow harmoniously together in one happy, unified society. It seemed like a good idea at the time but the outcome has been a disaster. Instead of growing together the different communities have tended to grow apart — the Muslim communities in particular.

Instead of integration there has been segregation. Lacking a clear and confident articulation of higher national values and purposes too many of the ethnic groupings that have grown up inside Britain have increasingly turned inward, building up feelings of alienation, and even hostility, to the wider society around them and the country in which they reside.

All sense of shared identity has faded. So much so that a senior Anglican bishop, himself of immigrant origins, has recently felt compelled to observe that there are now whole areas, already separate communities, which are now becoming "No Go" neighborhoods where non-Muslims fear to move about.

He, and other church leaders, are referring to areas where radical Islam reigns, where Shariah law and the powerful rule of the mosque prevail, where cultures and lifestyles have diverged more and more from the traditional British pattern and where young people feel, and are taught to feel, that they just do not belong.

The situation is even worse than that. It is now coming to light that under the mantle of multiculturalism, some radical Islamic groups in Britain have felt free to develop extreme ideological versions of their religion — in strong contrast to the gentle and tolerant Islam of the past — and then to export these ugly and violent doctrines round the world, including to such turbulent spots as Pakistan, Afghanistan and Iraq.

While the British authorities have benignly looked on, or turned a blind eye, ferocious philosophies of anti-democratic hate and destruction have been promulgated, only a few miles from Westminster, purportedly the home of democracy. Calls have been heard for the overthrow of Western governments as "the enemy," for laws and individual rights to be replaced by the higher will of the Islamic state, and for violent methods to replace peaceful debate.

In effect the good intentions of multiculturalism have allowed Britain to become not just a target for terrorists from elsewhere but the seedbed of theories to inspire and justify the activities of groups like al-Qaida and other dark forces.

As the dreadful implications of what has happened have dawned on the opinion-formers and policymakers, many prominent thinkers and religious leaders have joined the chorus of criticism of multiculturalism and pointed out its divisive consequences. Following rather than leading what has long been obvious to the general public, the authorities are now moving to deport the most intemperate advocates of violence, to keep a closer watch on Britain's growing number of mosques and to seek common cause with the more moderate and genuinely religious Muslim leaders, rather than the extremists who had almost completely hijacked the Muslim voice and cause throughout the country.

But what is their alternative? Is it outright assimilation — the requirement that all ethnic minorities should adopt fully and conform to the ways, laws and customs of the host society, on the principle of "when in Rome, do as the Romans do"? Should a higher and binding loyalty to the nation, its purposes and values, come first and foremost?

The trouble with this is that in the case of Britain these higher values and purposes are no longer by any means clear or agreed. Why, ask young Muslims, and their parents, should they give their obedience and commitment to laws made by remote and alien authorities and to a set of values that are empty of moral worth and anyway increasingly disregarded by the rest of society?

Meanwhile the wider public looks on with dismay at what is being bred in its domain. Envious glances are directed at more homogeneous societies, like Japan, which seem to be much freer of minority ethnic problems and the social and cultural disintegration that comes with them.

Demands grow for either a more strident assertion of Britishness, to which all newcomers and minorities must rigorously conform, or for the suppression of diversity run wild, or for the more rapid removal of the voices preaching hatred and disloyalty — or for all three approaches.

But of course it is far too late to change the direction of many of these trends. Britain is now the common home for an enormous variety of ethnic groups and cultures, estimates being that one in 10 of the British population were born overseas and that three percent are now Muslims.

Somehow they all have to learn not merely to live together but to shape the future together and to find a real focus for common loyalties and values. Unfortunately, the voice to define this point of focus, to explain that Britain is a cohesive and purposeful undertaking, worth belonging to, worth serving with pride and deserving of the loyalty of every resident and citizen, whatever their color, creed or religious faith, has yet to be found. Perhaps it will be soon.