"On Friday she got worse during a therapy session... and now she feels very sharp pain with almost any movement despite taking a strong painkiller," Yevgenia Tymoshenko told reporters in the eastern Ukrainian city of Kharkiv where Tymoshenko is serving her sentence.
Tymoshenko was jailed for seven years in 2011 on charges of overstepping her authority while prime minister by agreeing a gas deal with Russia, but has spent over a year in a hospital where she is being treated for back pain.
The opposition leader has dismissed the charges against her as politically motivated and orchestrated by Ukrainian President Viktor Yanukovych -- a long-time rival who won against her in the heated presidential polls in 2010.
"Her condition is very serious," said Yevgenia Tymoshenko, adding that her mother cannot walk without assistance and is only able to sit down for short periods of time.
In April the European Court of Human Rights ruled that detention of Tymoshenko was unlawful but threw out accusations of ill-treatment.
Her jailing has been condemned by Western governments as the result of selective persecution by the authorities and has led to a sharp deterioration in ties with the European Union, which Kiev wants to join.