"Our position remains the same: We will not attend political negotiations on Thursday with the Sudanese government until we hold our general conference on September 25-27," said a statement from the Sudan Liberation Movement (SLM) on Wednesday.
The group said it would still send a high-level delegation to the Nigerian capital Abuja "to inform the African Union and international mediators of our decision".
Besides Sudan's government and the SLM, the other rebel group Justice and Equality Movement (JEM) is due to attend the talks.
The negotiations had initially been scheduled for 24 August, but were put off at the request of the SLM, which said it first needed to hold a conference of its members.
The group had already warned last week that it might not be able to attend on the new date.
Rift among rebels
Analysts say the delay reflects a rift within the movement with one faction led by SLM leader Abdul Wahid Mohammed Nur in favour of taking part and another led by secretary-general Mani Arko Minawi wanting a postponement.
The SLM launched an armed rebellion in February 2003 in Darfur.
Up to 300,000 people have died and more than two million have fled their homes due to the fighting.