Tymoshenko is serving a seven-year prison sentence following a conviction on charges she abused her authority when helping to broker a natural gas deal with Russian energy company Gazprom.
Serhiy Vlasenko, a lawyer for Tymoshenko, was quoted by the Interfax news agency as saying his client said she started her hunger strike Friday. He said she was protesting the situation in Ukraine and her treatment while incarcerated.
"She has bruises on her arms and a big hematoma on the stomach, which is still visible, despite the fact that four days have passed," he was quoted as saying.
He denied allegations she's refused medical treatment while in prison.
Ihor Kolpaschykov, deputy head of the penal colony, denied claims that force was used against Tymoshenko. She was hospitalized last week for unspecified reasons though Kolpaschykov said she didn't complain about any injuries when she arrived.
He said he couldn't confirm the hunger strike.
"There is a refrigerator in Tymoshenko's cell," he was quoted by Interfax as saying. "She can eat without our being present."