Tsybizova, who had been a party member since its regional branch was founded, said the party's best days have already passed.
"Today there are only declarations and slogans, but no action is taking place," Tsybizova said in a statement published on the website V1.
Another reason for her withdrawal is the need to side with the party's position on important issues even when it went against her personal views.
"Previously, there was hope that the billions allocated to regional healthcare would be put to use accordingly. That hope died about six months ago … I felt ashamed for a long time, and left yesterday," the deputy said on her Facebook page.
The regional branch of United Russia said that it does not fear losing out as a result of the deputy's departure, and that "no one in the party had opposed her decision," Kommersant reported.
The newspaper, citing unidentified sources, said that Tsybizova intends to become a member of the Civic Platform party.
Tsybizova has been repeatedly elected as deputy of the Volgograd regional legislature and was a candidate for the State Duma. She was also part of the presidium of the political council of the regional party branch.
On May 13, another member of United Russia, Yury Kotler, left his position in the party's supreme council to become senior vice president of state bank VTB.