The 51-year-old Tymoshenko is on a hunger strike in protest at the prison's alleged mistreatment of her as she serves a seven-year sentence on charges of abusing her powers when she was prime minister.
Critics have described her case as politically motivated. They allege that President Viktor Yanukovych, Tymoshenko's rival in the 2010 presidential election, orchestrated her prosecution. Government officials have denied any claims of bias in the case.
Tymoshenko's aide Alexander Turchinov said on Friday: "She's been on a hunger strike for 15 days. Her life is in real danger."
But the Ukrainian prosecutor general, Viktor Pshonka, told reporters in Kiev that Ukrainian law did not allow prisoners to leave the country for medical treatment. He confirmed that two German doctors were examining Tymoshenko at her prison colony in eastern Ukraine.
Tymoshenko's allies were not told of the visit in advance.
German doctors from the Berlin-based Charite clinic also travelled to Ukraine last month to examine Tymoshenko. The doctors concluded that she was suffering from intense pain and needed urgent treatment in a specialist clinic. Tymoshenko's family said she was suffering from a herniated disc in her back.
The German government has said it believes Tymoshenko is unlikely to receive the treatment she needs in Ukraine and should be allowed to travel to Germany to get it.
But German foreign ministry spokesman Andreas Peschke said a quick solution was unlikely. "I want to dampen your expectations," he told reporters in Berlin. "Finding a solution here is complicated and will definitely take more time."
The Russian president-elect, Vladimir Putin, also offered help, suggesting that Russia would welcome Tymoshenko for treatment.
Putin was Tymoshenko's counterpart in signing a 2009 natural gas export deal that landed the Ukrainian politician in jail. Although Putin has stopped short of standing up for her, he has said there could not have been any violations of the law in the agreements they both signed.
On Thursday, the European Union president, Herman Van Rompuy, announced he would not travel to any of next month's Euro 2012 football matches in Ukraine in protest at Tymoshenko's treatment, joining other top officials such as European Commission president Jose Manuel Barroso, and the governments of Austria and Belgium. Putin has opposed such a boycott.
CNN