Ukraine has asked Germany to send a team of doctors to treat Yulia Tymoshenko after the opposition leader went on hunger strike in prison saying it was to protest her treatment in jail.
She has accused three men of punching her in the stomach on her way to hospital.
Ms Tymoshenko, 51, is said to have begun a hunger strike last Friday.
She was sentenced to prison for seven years for abuse of office in October after a trial condemned by Western states as politically motivated. She accuses President Viktor Yanukovych of seeking revenge.
The former prime minister has complained of severe back pain for some months.
The BBC reports doctors from a clinic in Berlin have already visited her in prison twice and the Ukraine government has asked Germany to conduct a new visit.