"I have already sent a statement to the United Russia leadership. We will take part in the elections independently," Vladimir Plotnikov said.
Commenting of Plotnikov's statement, First Deputy State Duma Speaker Oleg Morozov said: "I think our faction will treat Plotnikov's decision with understanding. The case is that today's legislation actually deprives his associates of the chance to participate in elections on a single list with United Russia, while saving the party's face," he said.
Morozov said United Russia does not consider the Agrarians' withdrawal a political move.
"This is a technical issue, and we will treat it with understanding," he said.
As the Kremlin-backed party that dominates the lower house of parliament, the State Duma, and regional governments, United Russia has consolidated its grip on most legislatures following regional elections March 11.
Only in the southern Stavropol Region was it outperformed by the newly formed Just Russia party.
A new Kremlin project designed to counterbalance the dominant "party of power" and poach votes from the Communists, Just Russia proactively used welfare policies in its campaign.
The legislature polls are seen as a "rehearsal" ahead of elections to the State Duma in December and a presidential poll in March 2008.
Local polls were held in the wake of legislative amendments that raised the vote threshold for the State Duma, increased membership requirements for political parties, abolished the 20% threshold for voter turnout and canceled the "against all candidates" option on election ballots.
Rights groups have slammed the changes as undemocratic, but the Kremlin said they were needed to safeguard the development of Russian democracy from irresponsible marginal parties.
