Preliminary hearings have started in the trial against Tymoshenko on tax evasion and attempted theft charges related to her activities as the head of a major gas trading company in the 1990s.
The imprisonment last October of Tymoshenko, the fiercest foe of President Viktor Yanukovich, has soured Ukraine's ties with the European Union and the United States, which saw the case as an example of "selective justice".
Despite Western pressure for her release, Ukrainian prosecutors have charged her with tax evasion and are also investigating what they say is her possible involvement in the 1996 contract killing of an influential businessman and parliament deputy.
In an official statement published by the state prison service, Tymoshenko, who had earlier denied any wrongdoing, refused to attend the opening hearing.
"I am asking [the court] to hold the preliminary hearing in my absence due to my poor health," Tymoshenko said.
Tymoshenko, 51, has been suffering from back pains for months, her family and lawyers say, and has been unable to walk at times.
The authorities have proposed to move her to a state hospital in the city of Kharkiv where her prison is located but Tymoshenko has sought the advice of German doctors who are now in Ukraine to inspect the proposed facility.
Ukrainian and German government officials meanwhile say their governments are in talks with a view to her possibly going to Germany for treatment, but it is not clear if Tymoshenko herself would agree to go.
Tymoshenko was convicted in October of abusing her power as prime minister in forcing through a 2009 gas deal with Russia which, Yanukovich's government says, ran against national interests and made vital imports exorbitantly expensive.
She dismissed the verdict as part of Yanukovich's personal vendetta.
Tymoshenko gained international prominence as one of the leaders of the 2004 Orange Revolution which doomed Yanukovich's first bid for the presidency, after which she went on to serve twice as prime minister.
But she lost the 2010 presidential election to Yanukovich. After his rise to power, Tymoshenko and a number of her allies in opposition faced corruption-related charges in what she has described as a crackdown on opposition.
Tymoshenko is challenging her initial conviction in the European Court of Human Rights. The European Union has warned Kyiv that its member states will not ratify a milestone Ukraine-EU association agreement while she is in prison.