"At the congress I will resign as the party's leader and will become a rank-and-file member of the SPS next week," said Nemtsov. He did not explain the reasons behind the decision.
At the same time he said that the discussions at the congress would be difficult. Among other topics, its participants will analyse the situation in the run-off of the coming presidential elections.
"Some of the SPS members support Irina Khakamada, others call for voting against all, and still others support the incumbent head of the state," said Boris Nemtsov.
"The discussions will be serious and deep," forecasts Nemtsov.
SPS co-chairman Irina Khakamada nominated herself for the presidency at the end of December.
The presidential elections have been scheduled for March 14, this year.