Boris Nemtsov, a member of the SPS political council, said disagreements over what had been in the past and personal misunderstanding impede the unification of both parties.
"I could state with regret that the parties won't be able to unite," Nemtsov said.
The SPS, which supports western-style capitalism and is associated with the free market reforms and privatization of the 1990s under the presidency of Boris Yeltsin, failed to reach the 5% threshold in the 2003 elections needed to take up seats in the State Duma. It has two seats in the 450-seat Duma, all from single-mandate districts, compared with the 32 it took in 1999.
In 2003, Yabloko won only four seats in Russia's 450-seat Duma, and all from single-mandate districts. Both parties won less than the 5% of the vote necessary to get deputies elected from party lists.
