"If south Sudan secedes, we will change the constitution and at that time there will be no time to speak of diversity of culture and ethnicity," President Omar Bashir told supporters at a rally in the eastern city of Gedaref.

"Shariah (Islamic law) and Islam will be the main source for the constitution, Islam the official religion and Arabic the official language," he said.

The plebiscite on whether to declare independence in the south, where most follow traditional beliefs and Christianity, was promised in a 2005 peace deal that ended decades of war between it and the mainly Muslim north.

The accord set up an interim constitution that limited the application of Shariah to the north and recognized "the cultural and social diversity of the Sudanese people."

Analysts expect most southerners to choose independence in the poll, scheduled to start Jan. 9 and last for a week.

Bashir's statements will add to concerns about how hundreds of thousands of southerners living in the north might be treated after a split. Ministers from Bashir's National Congress Party (NCP) have warned that southerners would lose northern citizenship rights and might even be denied treatment in hospitals.

Northern and southern leaders are locked in negotiations over a number of issues, including the sharing out of oil revenues and the citizenship of their people after separation.

Bashir also defended police shown lashing a woman in a film that appeared on the video-sharing website YouTube. "If she is lashed according to Shariah law there is no investigation. Why are some people ashamed? This is Shariah," he said.

Senior NCP member Nafie Ali Nafie said Thursday efforts to keep the country united had failed, in the first acknowledgment from the northern elite that the south would probably secede.

Floggings carried out under Islamic law are almost a daily punishment in northern Sudan for crimes including drinking alcohol and adultery.