Tymoshenko is serving a seven-year prison sentence on charges she abused her authority when, as prime minister in 2009, she helped broker a natural gas deal with Russian energy company Gazprom. She held a brief hunger strike last year to protest prison conditions and recently launched a campaign of civil disobedience.
Serhiy Vlasenko, her lawyer, said his client was found unresponsive.
"When I walked in (to her cell room) I thought she was dead," he said in a statement on her website. "I'm not a specialist, but the situation is critical."
He didn't provide details about any particular ailments, though she's complained about her health in the past.
Vlasenko added that a district court in the country had ignored a motion to close the case against his client. Her supporters, including those from Western powers, believe the charges against the leader of the country's Orange Revolution in 2004 are politically motivated.
Her party took second in parliamentary elections held in October. European observers said those elections were a step backward for a country that has tried to move closer to the European Union.