MOSCOW — The Orange coalition of Western-leaning political factions in Ukraine’s Parliament disintegrated Tuesday, in what could be the first stage in an effort to unseat Prime Minister Yulia V. Tymoshenko.

Ms. Tymoshenko, who led the coalition, immediately labeled the dissolution illegal, and vowed to continue to lead the “democratic team.”

She has so far rebuffed calls for her resignation from Viktor F. Yanukovich, who defeated her in a bitterly contested presidential election last month. But the collapse of the majority coalition in Parliament, which Ms. Tymoshenko led, could seriously undermine her ability to hang on, analysts said.

“I declare that the coalition no longer exists,” the Parliament’s speaker, Volodymyr M. Lytvyn, said in remarks broadcast on Russian television. Mr. Lytvyn said that the Our Ukraine faction and his own bloc had pulled out of the coalition, leaving it without the 226 seats needed to hold a majority.