Gabriel Changson Chang, information minister in the southern government, confirmed to AFP news agency: "We have deferred the census until sometime this year.
"We feel that if we were to carry out the census now, it would not achieve the objectives for which it was intended, so we need more time to work on some of the issues."
The National Congress party (NCP), which shares the central government with the south's Sudan People's Liberation Movement (SPLM), has condemned the boycott after an emergency meeting late on Sunday.
Omar Al-Bashir, Sudan's president and head of the National Congress party, chaired the meeting, which the official SUNA news agency said was to ask the SPLM to reverse its decision.
Kamal Obeid, NCP's spokesman, told SUNA there could be no justification for the last-minute postponement, and expressed concern about the knock-on effect on the timetable for elections, currently scheduled for 2009.
Salva Kiir, first vice-president of South Sudan, is to arrive in Khartoum on Monday to have crisis talks with al-Bashir, one source in the government said.
The census is to prepare for voter registration. Its results will also redraw or confirm the ratio of central power-sharing between north and south.
Chang said that various disputes with the north, which he said have not been resolved under the slipping timetable to implement the 2005 peace deal, must be addressed or completed before the south approves the population count.
"There is no peaceful atmosphere. Why don't we talk about those pending issues, the border issues, Abyei [an oil-rich area claimed by both north and south where tensions have risen in recent weeks], Darfur?" he said.
|A soldier from the ex-rebel Sudan People's|
Liberation Movement takes a break [AFP]
The south cited the north's veto of ethnicity and religion on the census questionnaires as a reason for the delay, although UN overseers had said both sides agreed on the compromise formula of northern or southern Sudanese.
"Sudan is an African country because the majority of Sudanese people are Africans, which contradicts directly the claim we are an Arab nation. This can only be proved through an objective exercise like the census," said Chang.
The Arab domination of power in what is Africa's largest country was a major reason for the two-decade civil war between north and south, as well as for the separate five-year conflict in western Sudan's Darfur region.
Officials have said parts of Darfur under rebel control and the Egyptian-occupied Halayib triangle in the northeast will be excluded from what will be Sudan's fifth census since independence in 1956.
Chang said two million displaced southerners living in the north must be given the clear option to be repatriated to the under-developed south before the census, otherwise the count would disadvantage them.
"The government of south Sudan will take measures to make sure they are repatriated before the census is carried out... There is undue influence in Khartoum in preventing the IDPs (internally displaced persons) from coming back to the south," he said.
"The power-sharing, which was agreed on in the CPA, would be reviewed after the census and once our population reduces from what it is assumed to be, then automatically the power sharing is going to be revised downwards," he said.
Human Rights Watch says militia attacks in disputed border areas may reflect an attempt by Khartoum to skew census registration.
"We'll make sure that the process of demarcation is speeded up and completed before we go for the census," Chang said.
Yasin Haj Abdin, the director of the central bureau of statistics, the civil servant in charge of the population count, issued a statement denying it was necessary for IDP or border issues to be resolved before the census.
He said the prospect of IDPs returning and new field work being conducted to map the expected returnees by the end of the year would be impossible because of the rainy season, which has already begun in parts of the south.