By Robert Pigott
Religious affairs correspondent, BBC News
Dr Nazir-Ali says multi-culturalism has created further segregation of society
The marginalisation of Christianity in British life has created a moral vacuum that radical Islam is threatening to fill, a senior bishop has warned.
The Bishop of Rochester said Christianity had created a British identity imbued with values such as liberty and freedom of conscience.
Dr Michael Nazir-Ali said this had been undermined during and since the 1960s.
But Mohammed Shafiq of the Ramadhan Foundation said radical Islam was not representative of British Muslims.
Mr Shafiq said any moral vacuum in Britain was not caused by an eclipse of Christianity, so much as the failure of the Church to transmit Christian values to people.
'What is Britishness?'
Writing in the first edition of the current affairs magazine, Standpoint, Dr Nazir-Ali said the decline of Christianity produced a lack of "transcendental principles" which has left the door open for the "comprehensive" claims of radical Islam.
The bishop, who was born in Pakistan of Christian parents, said Christianity had knitted together a "rabble of mutually hostile tribes" to create British identity.
But Dr Nazir-Ali said the loss of what he called the Christian consensus had led to the breakdown of the family, abuse of drugs and alcohol, and a loss of respect for other people.
He said the marginalisation of Christianity had happened just as large numbers of people of other faiths arrived in Britain.
He argues that the policy of multi-culturalism, far from uniting society, has led to its further segregation by emphasising religious distinctiveness rather than integration.
Dr Nazir-Ali said it had raised the question: "What now is Britishness?"
He claims it was the weakened British identity that once opened the way to a challenge - successfully fought off - from Marxism.
"But we are now confronted by another equally serious ideology," he said, "that of radical Islamism, which also claims to be comprehensive in scope.
"What resources do we have to face yet another ideological battle?"
He said the government had been able to come up only with "thin" values - such as tolerance, decency and fairness.
What are needed, he insisted, were the "transcendental principles" of Christianity - the sort of fundamental issues that are raised when people consider what it is to be human, and life and death questions such as abortion, euthanasia and stem-cell research.
But are such beliefs and principles not now "free-standing" - possible without any reference to Christianity?
Dr Nazir-Ali claims that disengaging the values that helped to create a British identity from the religion that inspired them has already helped to make them less plausible to people.
He does not think that other religions make adequate substitutes for Christianity.
Dr Nazir-Ali said radical Islam emphasised the solidarity of the worldwide community of faithful Muslims against the freedom of the individual.
He said other religions also gave more importance to "public piety" than internal spirituality.
The bishop also said religion was bound to play a role in public life, and its leaders would seek to influence it.
Conversion of Muslims
But he opposed the idea - suggested by the Archbishop of Canterbury Rowan Williams - that elements of Sharia should be incorporated into British law (Dr Williams said it was "inevitable").
Dr Nazir-Ali said: "It would be best if religious law in its application were left to the communities and to the free obedience of their members.
"Public law, however, should continue to provide overarching protection for all."
Dr Nazir-Ali believes the Church itself should be more robust in standing up for its core beliefs.
There is a small problem of some idiots who want to use political violence, but it's forbidden by Islam
He is one of three Church of England bishops to back an initiative by a traditionalist Anglican to commit the Church to work explicitly for the conversion of Muslims.
Paul Eddy's motion, which has received sufficient support to guarantee a hearing at this summer's meeting of the Church's synod, would call on the Church to proclaim Christianity as the only route to ultimate salvation, a stance likely to alienate many Muslims.
Mr Eddy says he has come under intense pressure to withdraw the proposal.
But Mr Shafiq of the Ramadhan Foundation said Islam had nothing to fear from a Church that was clear about its aim to convert Muslims.
He said: "All faiths see themselves as the true path, but at the moment the traffic is coming the other way with Christians converting to Islam every day because they know it's a religion of compassion".
He also rejected Dr Nazir-Ali's central claims about the influence on Britain of Islam and multiculturalism.
Mr Shafiq said it was wrong to imply the presence of a mass of radical Muslims seeking to impose their views.
He said: "There is a small problem of some idiots who want to use political violence, but it's forbidden by Islam".
He also said the bishop was wrong to dismiss values such as respect, toleration and fairness as "thin".
Defending the policy of multi-culturalism, Mr Shafiq said: "We are a multi-faith society, and we can share values like respect, tolerance and free speech.
"The fact that we have many faiths enriches the country."