Ahead of a new round of peace talks in Nigeria later this month, the Sudan Liberation Movement (SLM) said it would not take part unless fellow rebels from the Justice and Equality Movement (JEM) were also allowed to take part.
Talks have been scheduled to take place in Abuja, Nigeria on 21 October.
Khartoum "has no right to name which of the opposition movements can take part in the talks but can name its own delegation," SLM spokesman Mahjub Husayn told the Akhbar Al-Yawm newspaper from his base in London on Sunday.
The Sudanese government has accused the JEM of complicity in an alleged coup attempt in Khartoum last month by the Popular Congress party of jailed opposition leader Hasan al-Turabi and rejected it as a negotiating partner.
But the rebels, and other groups opposed to Khartoum's military-backed regime, say that the alleged coup attempt never took place and was a mere government ploy to divert attention from its mounting diplomatic woes.
Al-Turabi's Popular Congress is
accused of attempting a coup
The SLM spokesman expressed concern that Khartoum was set to retain hardline Agriculture Minister Majdhub al-Khalifa Ahmad, for the next round of negotiations.
The current team "lacked the political vision needed for the negotiations," Husayn said.
The agriculture minister has been outspoken in rejecting mounting international pressure for a genuinely federal system to address the grievances of Sudan's non-Arab minorities in the north as well as the south.
On Thursday he was quoted as saying that Khartoum "does not speak at the moment about autonomy to any region in the north," backtracking on pledges by other ministers that self-rule for Darfur's indigenous minorities was on the table.
A government official, meanwhile, accused the SLM as well as the JEM of "conspiring" with the Popular Congress and charged that representatives of both factions had abused their positions on an African Union ceasefire observer mission to further their uprising.
Spread of uprising
Officials in other regions of Sudan voiced concern that the Darfur uprising risked spreading, after a rebel attack in West Kordofan state on Friday.
Almost 200,000 people have
taken refuge in Chad
"Fear grips North Kordofan in anticipation of raids by the Darfur rebels," ruling National Congress official Abd Al-Wahab Hasan Husayn told Akhbar Al-Yawm.
"Attacking North Kordofan poses a threat to the capital Khartoum."
In eastern Sudan, the governor of Kassala state, Faruq Hasan Muhammad Nur, said attention had turned to his state after the failure of the alleged coup attempt.
"After its recent failure in Khartoum, the Popular Congress was planning to execute a subversive plot in Kassala," Nur told the pro-government Sudanese Media Centre.
As many as 50,000 people have died and nearly 200,000 taken refuge in neighbouring Chad since Khartoum started its bloody clampdown on the rebels and minority civilians suspected of supporting them.
Despite two UN Security Council resolutions and an agreement with UN Secretary General Kofi Annan, Khartoum has yet to meet demands to stem what the United Nations describes as the world's worst current humanitarian crisis.