"The executive bureau of the National Coordination Body for Democratic Change (NCBDC) has decided to reject participation in the Geneva conference in the conditions... It is currently being prepared in," the group said.
"The United Nations and the two sponsoring states (the United States and Russia) called on the Coalition to form a delegation that represented Syria's opposition in a balanced way, under the Coalition's umbrella," it added.
The NCBDC said it was only "informed of these facts on December 13 when it received a copy of an invitation letter addressed to the president of the National Coalition," Ahmad Jarba.
The NCBDC accused Russia of "backing down" on its earlier position that three separate opposition groups should be invited to the talks.
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"How can a single Syrian opposition delegation be expected to have a solid position... If the Coalition, which the two sponsor states have chosen as a representative of the Syrian people, has not yet decided whether it will attend?" it asked.
The Coalition has said it will not decide whether to attend until a meeting of its General Assembly on January 17, just five days before the Geneva II conference is due to open.
The NCBDC cited the regime's "arbitrary shelling" of opposition areas, its failure to release women and children detained in its jails, the blocking of relief operations, and international sanctions against Syria as further reasons for its decision to boycott the peace talks.