The idea that the culture of different ethnic groups should be nurtured and celebrated, as the basis for social harmony, has been the cornerstone of Britain's attitude to its minority communities for more than three decades. Multiculturalism was never popular with the Little Englanders of the right. But in recent years it has increasingly been attacked by the liberal left.

It is the critique of these so-called "muscular liberals" that David Cameron has adopted in his surprisingly hardline speech.

The shift began after the traumatic terrorist attacks of 9/11. Many of those who like to think of themselves as liberals then began exhibiting a new intolerance, demanding that minorities should assimilate more. Multiculturalism must not be allowed, as the Prime Minister now says, to encourage different cultures to live separate lives, sometimes behaving in ways that run counter to "our" values.

But he, like so many illiberal liberals, has resorted to caricature to make his case, citing practices like forced marriage as an example, a practice that is, as distinct from arranged marriages, generally condemned as unacceptable in most minority communities.

Some of Mr Cameron's practical measures will be uncontroversial among minority groups. Few will object to a requirement that immigrants should learn English or that the school curriculum should celebrate "a common culture", if it is common. Ethnic communities with unusually high levels of youth unemployment will welcome the National Citizen Service to bring 16-year-olds from different backgrounds to live and work together.

But they will bridle at the unnecessarily confrontational tone with which he warns that "Europe needs to wake up to what is happening in our own countries". Withdrawing public cash from Muslim organisations which do not subscribe to particular values on "integration or separatism" sends an overly dogmatic message. Will they be required to endorse British foreign policy in Afghanistan or Israel? Or will he, like other right-wing governments in Europe, introduce prohibitions on Islamic dress?

The truth is that Britain has had too little multiculturalism. And serious multiculturalism requires policies such as the "racial identity nurturing" to be found in schools with large black populations. This shows black kids other options than rap music, boxing, drug-dealing or other crime. It also shows them black physicists, doctors and businessmen as role models who motivate pupils to do much better at school. Lifting ethnic minorities out of poverty is the way forward, not finger-wagging.

Such kids form relationships that spill over into the wider community. One school in Moss Side in Manchester has 37 nationalities. Black children there do Irish dancing, and white kids play in a Jamaican steel band. There are similar projects in the Asian communities in Oldham and Leicester.

Multiculturalism is about creating a highest common factor society. David Cameron's approach will doom us to a lowest common denominator one.