Tymoshenko was quoted by Ukraine's State Penitentiary Service on December 27 as saying that her German doctors had recommended that she not attend the trial.
Tymoshenko, a bitter rival of President Viktor Yanukovych, is serving a seven-year sentence after being convicted of abuse of power.
She has been treated for back pain in a hospital in Kharkiv for more than a year.
The tax evasion trial focuses on allegations Tymoshenko evaded millions of dollars in taxes while leading a private energy company in the 1990s.
It has been postponed more than 10 times since April 2012 due to her refusal to take part.
The EU considers Tymoshenko's conviction to be politically motivated.
Her release to receive medical treatment in Germany had been a key condition for Kyiv to sign an Association Agreement with the bloc.