Azerbaijan's Central Election Commission (MSK) has refused to register the united opposition's single presidential candidate, Rustam Ibragimbekov.
The commission said its decision on August 27 was based on the fact that Ibragimbekov has dual citizenship and is not a permanent resident of Azerbaijan.
The 74-year-old Oscar-winning screenwriter was nominated as the opposition's candidate earlier this month.
He has been outside of Azerbaijan and working on giving up his Russian citizenship.
Last week, Azerbaijan's united opposition nominated 61-year-old historian and former lawmaker Camil Hasanli to be its alternate candidate.
Azerbaijan's presidential election is scheduled for October 9.
Incumbent Ilham Aliyev, who has held power since succeeding his father in 2003, has been nominated by the ruling New Azerbaijan Party as its presidential candidate and was officially registered by the MSK this month.