MP Serhii Vlasenko told the Ukrainian news agency UNA that Tymoshenko stopped eating April 20 at the Kachanivska women's penal colony.
In a related development, he said prison guards reportedly kicked her in the stomach while taking her to a clinic.
The former leader suffers from a severe spinal condition, but Tymoshenko refused treatment at the Kharkiv clinic because she doesn't trust the government-appointed doctors there, Vlasenko said.
Tymoshenko, 51, is serving a seven-year sentence for abusing her powers while negotiating gas contracts with Russia.
Her guilty verdict was strongly condemned by the West as politically motivated.
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