Kasyanov, a critic of the Kremlin, is seen as a likely opposition candidate in 2008 presidential elections.
But reports say Kasyanov was badly defeated at a Democratic Party congress in Moscow today, with the vast majority of 225 delegates voting for Andrei Bogdanov instead. The two men have been vying for control over the party for weeks.
Kasyanov today chaired a rival party convention in another part of the Russian capital. The 150 delegates in attendance elected him their informal leader and called for the creation of a new party.
Kasyanov was removed from office in February of last year. Yesterday, he accused the Kremlin of inspiring his opponents within the Democratic Party.
2005 In Pictures
A slideshow of images related to the top news stories of 2005.