Ukraine cancelled a summit for Central and East European leaders on Tuesday after a string of heads of state refused to attend the event over the treatment in jail of former Prime Minister Yulia Tymoshenko.
In the latest blow to President Viktor Yanukovych, Ukraine's foreign ministry said the summit scheduled for Friday and Saturday in the Black Sea resort of Yalta would not go ahead because of the boycott.
"The event will take place at a later date that will be established via diplomatic channels," said a spokesman.
Mr Yanukovych's government has come under growing pressure since Mrs Tymoshenko, 51, was allegedly beaten last month at the penal colony in Kharkiv, eastern Ukraine, where she is serving out a seven-year sentence for abuse of office. The former prime minister has been on hunger strike since the incident on April 20.
Her jailing in October for supposedly signing a damaging 2009 gas deal with Russia was widely seen as political revenge by Mr Yanukovych, her chief political foe for almost a decade.
The presidents of Germany, Austria, Italy, Croatia, Estonia, Slovenia, Bulgaria and the Czech Republic decided not to travel to the Yalta summit after news broke of Mrs Tymoshenko's ordeal.
Several European governments have also said they will boycott matches in Ukraine during the Euro 2012 football tournament to be held there from June 8 if they see no progress in her case.
In a separate development on Tuesday, Mrs Tymoshenko refused to begin treatment for a long term back problem at a Kharkiv clinic, under the supervision of German doctors.
The politician had indicated last week that she was happy to be cared for there in the presence of German specialists because she was suspicious of government-appointed local doctors. The state penitentiary service said she needed more time to consult her lawyers and may yet begin the treatment on Wednesday.
Mrs Tymoshenko was apparently left with bruises on her arm and stomach after prison guards manhandled her to the same clinic against her will last month. Photographs of the injuries were published online, causing disquiet in Europe and the United States.
Ukrainian prosecutors say Mrs Tymoshenko was not punched, as she claims, but may have "bumped into blunt objects". They have refused to open a criminal investigation.
The politician has not yet decided whether she will continue her hunger strike after beginning treatment, her lawyers said on Tuesday.