South Sudan's new constitution is to be adopted when the region becomes independent on 9 July.
However, opposition politicians are furious to learn that the draft document grants President Salva Kiir another four years in power.
The draft charter also lays claim to Abyei, which is a fiercely contested oil-producing area straddling north and south Sudan.
Its disputed ownership is seen by analysts as having the potential to lead the country back to civil war.
North and South Sudan fought each other for all but a few years since 1955 over differences in ethnicity, ideology, religion and oil.
The conflict claimed at least two million lives and destabilised much of the region.
Southern Sudanese voted in January to separate from the north and form a new nation, a referendum promised to them as part of a 2005 peace deal which ended decades of civil war.
The draft, which calls itself the 'Transitional Constitution' according to Reuters, is set to be ratified by parliament and will form a temporary charter from independence until a permanent constitution is drafted.
But it outlines steps which opposition leaders say amount to a betrayal.
'They were supposed to be making only minor changes to the Interim Constitution but they have written a new constitution ... (inserting detail) that was meant to be decided (later) in the constitutional conference,' said opposition leader Lam Akol, referring to an inclusive process promised by the south's dominant SPLM party to draft a post-independence constitution.
'They said they would just be removing redundant language that reflects a regional constitution becoming a national one, but they have set the president's term without mentioning limits,' Mr Akol added.
Similar fears are shared by more radical opposition.
At least seven rebel militia are fighting to overthrow the semi-autonomous southern government which they say only serves its own interests, ignoring minority tribes and rural communities in the highly underdeveloped region.
'There will be no review process that will change anything, opposition and civil society won't be able to make changes (to the constitution),' said Bol Gatkouth, a spokesman for one of the rebel groups.
The government says the permanent constitution will be fairly debated at a conference which the draft said must include opposition parties and civil society among others.
The national interim constitution drafted after the 2005 peace deal set the president's term at five years with a two-term limit, and Salva Kiir won a new mandate in elections last April.
The new South Sudan draft grants a four year tenure 'commencing from July 9, 2011' but specifies no limit to how many terms the president can serve.
Abyei
An Abyei referendum on whether to join the north or south was meant to run parallel to the January vote, but it did not take place. Talks on Abyei's future have stalled.
Both the north and south have been building up forces in the area and arming them with weapons such as rocket-propelled grenades and machine guns, according to satellite images and the United Nations.
The northern ruling NCP said the claim to Abyei in the constitution was meaningless and the south would not be able to implement it on the ground.
'The Abyei problem is not resolved yet,' said senior NCP official Rabie Abdelati. 'I think this provision will not be respected or considered by our government.'
The Transitional Constitution draft said Juba would remain the capital city and the establishment of a central bank would oversee the adoption of a new currency.
The Sudanese pound in Juba has steadily weakened on the black market since the January vote, this week reaching 3.3 to the dollar as many fear it may not be redeemable after July.
