A Just Russia has kicked out Gennady Gudkov and his son Dmitry Gudkov from the party after both refused to quit their membership in the opposition's Coordination Council.
The party bureau made the decision late Wednesday, arguing that both members had damaged the party's political interests and image, Interfax reported.
Party leader Sergei Mironov said that membership in the opposition council breached party rules.
"They want to have both but they can only have one or the other," he said in comments shown on state television.
The decision means that Just Russia will lose two of its most high-profile and outspoken members. It also throws into doubt the party's role as the most opposition-leaning force in the State Duma.
The Gudkovs have come under increasing pressure in recent months for their open support for anti-government street protests.
Gennady Gudkov, a former career KGB officer who has been a member of parliament since 2001, was excluded from the Duma last fall. While the official reason was that he continued running a private security business in breach of parliamentary rules, most observers saw his ouster as a punishment for his political views.
His son Dmitry has come under fire for a recent trip to the United States where he spoke at a Freedom House forum. Nationalist firebrand Vladimir Zhirinovsky demanded Gudkov's arrest "for betraying the country."
A Just Russia was formed in 2006 under Kremlin tutelage as a moderate left-wing party aimed against the Communists.