The National Coalition, an opposition umbrella group increasingly at odds with rebels on the ground, has previously said it would attend the Geneva talks slated for January 22 but with conditions.
Crucially, it insists that President Bashar al-Assad play no role in Syria's future -- a demand strongly rejected by Damascus.
"A final decision will be taken during a meeting of the Coalition in mid-December in Istanbul," opposition member George Sabra told AFP in the Qatari capital, Doha.
"I have doubts that the conference will take place," said Sabra, who heads the Syrian National Council (SNC), the largest member of the National Coalition.
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The SNC has in the past said it would not attend the Geneva 2 talks.
"No one will dare go to Geneva without consulting with the forces on the ground who retain the real power" to negotiate, he said.
The Geneva conference is aimed at ending the nearly three-year-old civil war, a bloody stalemate which has killed an estimated 126,000 people and driven millions from their homes.
But a newly-formed rebel alliance has said it wants to replace the Syrian regime with an Islamic state and a coalition of 19 Islamist groups fighting to topple Assad have rejected the talks outright.