However, this will not happen soon, he added.
"We [SPS and Yabloko] have agreed to unite and make a common list for the next parliamentary elections. This is a real breakthrough," Mr. Nadezhdin said.
But the talks are far from completion, he noted.
Mr. Nadezhdin did not disclose the details of the talks which are supported by the Committee-2008.
Lyudmila Alekseyeva, a member of the Committee-2008 and cochairman of the All-Russia Civil Congress which called on liberals to unite at its first congress, also highly assessed the readiness of the two opposed parties for compromise.
"I attended sessions of the committee and thought it was hopeless. However, considerable progress has been achieved - both parties agreed to give up titles and replace leaders," Ms. Alekseyeva said at a RIA Novosti press conference.
The Yabloko demands are considered debatable. The SPS is ready to unite with almost no conditions.
"There is only one condition - the discussion of the role of Anatoly Chubais and Grigory Yavlinsky in history should be stopped," Mr. Nadezhdin stressed.
In his opinion, the registration of Irina Khakamada's or Vladimir Ryzhkov's political parties will not change the general trend of the formation of a common liberal list for the State Duma elections in 2007 and will be just one more reason during the talks.