As a third Nigerian state moves towards officially implementing strict Koranic laws, the country's leading Christian organization has threatened to mount a legal challenge, claiming that Koranic law is unconstitutional and jeopardizes the unity of the country.

The Christian Association of Nigeria (CAN), an umbrella body for the country's Roman Catholic and Protestant churches, said it had written to the Nigerian attorney general asking him to challenge the imposition, six weeks ago, of shari'a law in Zamfara state, northern Nigeria. (Shari'a is the sacred law of Islam, embracing all aspects of a Muslim's life.)

Since the decision by Zamfara state, two more northern Nigerian states, Kano and Sokoto, which are bigger and more influential than Zamfara, have taken steps to adopt Islamic law. In Sokoto, the authorities last week announced a ban on alcohol and prostitution while Kano, which has a population of about five million, announced the full enforcement of shari'a.

In a separate move, the southern state of Rivers, which has seen street protests against the Islamist moves, has threatened to declare itself a Christian state in protest.

CAN's general secretary, Charles Obasola (C.O.) Williams, said in Lagos: "There seems to be a sinister motive to this shari'a decision. We have written to the attorney general calling on him to take action against Zamfara. If he does not, we will consider taking our own legal action."

You cannot have two systems of law—common law [the legal system introduced in Nigeria by Britain] and shari'a. Many states in the north are now talking about shari'a but this runs counter to the well-integrated nature of many parts of the country where there is intermarriage and peaceful cohabitation. We know shari'a ...