Chad's military leader will sign a deal on Monday with more than 40 opposition groups to launch national peace talks, but the central African country's main rebel outfit has refused to take part.
Under the accord, to be signed in the Qatari capital Doha, talks aimed at paving the way for a presidential election will start on August 20.
Since March, Qatar has been mediating between opposition groups and the military government of Mahamat Idriss Deby Itno, a general who seized power after his father died in a battle with rebels last year.
But the Front for Change and Concord in Chad (FACT), the main rebel group, said it would not sign the deal despite last-minute efforts by Qatar's mediators.
In a statement released in Doha, FACT said it "rejects the accord that will be put to signatories on Monday".
It added that participants in the national dialogue would not be treated equally and demanded a new committee be set up to organise the talks, as well as the release of rebel prisoners in government prisons.
"However FACT remains available for dialogue anywhere and anytime," added the statement from the group, which is estimated to have between 1,500 and 2,000 fighters.
FACT fighters led the rebel offensive in which Deby's father, Idriss Deby Itno, who had been president for 30 years, was killed.
Chad has had little stability since its independence in 1960, and the coming talks are being widely watched, as the country is seen as a key ally in international efforts to counter Islamic extremists fighting around the region.
Qatar's foreign ministry said the talks, to be held in Chad's capital, N'Djamena, would seek "inclusive national reconciliation".
Some 42 of the 47 groups represented in the Doha talks will sign the accord, officials said.
Talks will 'struggle' without FACT
The Doha accord commits signatories to a ceasefire during the N'Djamena talks.
The military government has also guaranteed the safety of rebel leaders who attend the talks.
Qatar had wanted FACT leader Mahamat Mahdi Ali to leave his desert camp in Libya to attend the signing.
But FACT and other groups say the guarantees were not strong enough. They have also demanded that Deby pledge in advance that he will not stand in any election.
Deby, 38, promised elections in 18 months when he seized power in April last year.
But his junta has retained the power to extend its "transitional" rule by 18 months. Deby faces pressure, however, from France, the European Union and the African Union to meet the October deadline.
"Getting this many groups to sign the accord is a good launchpad for the national dialogue, but it will struggle without outfits such as FACT," said the head of one political group that has agreed to sign on Monday.
The Chadian government has said that more than 1,300 representatives of rebel groups, civil society, trade unions, political parties and government officials will attend the N'Djamena talks.