Omar al-Bashir, the Sudanese president, said the country would adopt an Islamic constitution if the south split away in next month's referendum, in a speech today in which he also defended police filming a woman being flogged.

"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," Bashir told supporters at a rally in the eastern city of Gedaref.

"Sharia [Islamic law] and Islam will be the main source for the constitution, Islam the official religion and Arabic the official language," he said.

An official from south Sudan's main party criticised Bashir's stance, saying it would encourage discrimination against minorities in the mainly Muslim north and deepen the country's international isolation.

The referendum in south Sudan, where most follow indigenous beliefs and Christianity, on whether to declare independence is scheduled to start on 9 January.

The vote was promised in a 2005 peace deal that ended a civil war between north and south, and set up an interim constitution which limited sharia to the north and recognised "the cultural and social diversity of the Sudanese people".

Analysts expect most southerners to choose independence in the poll, due to last for a week. Yasir Arman, from the Sudan People's Liberation Movement (SPLM), said Bashir's statements would encourage repression in the north. "This type of discourse is preparing the ground for a police state. The north, whether alone or with the south, is an extremely diverse place."

Arman said it was the north's hardline stance that had pushed southerners towards separation. "If it [the north] continues like this it will encourage other areas like Darfur, the Nuba mountains and eastern Sudan to walk out as well," he added, referring to areas on the peripheries of northern Sudan. "It will also result in Sudan having worse relations with the outside world."

Southern leaders have said they are worried about how hundreds of thousands of southerners living in the north might be treated after a split.

Arman, Bashir's main challenger in the presidential elections in April, is from the northern sector of the SPLM. He said his group would form a separate opposition party inside the north if the south seceded.

Bashir also defended police shown lashing a woman in footage that appeared on YouTube. "If she is lashed according to sharia law there is no investigation. Why are some people ashamed? This is sharia," he said.

Senior NCP member Nafie Ali Nafie said on Thursday that efforts to keep the country united had failed, in the first acknowledgement 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.