The move, which follows allegations of Tymoshenko's beating in prison, highlights Ukraine's growing isolation and casts doubts on Kiev's plans to improve its image abroad by hosting the European soccer championship next month.
The presidents of Germany, Austria, Italy, Croatia, Estonia, Slovenia, Bulgaria and the Czech Republic have all said they would not attend the May 11-12 gathering in the Black Sea resort of Yalta to be hosted by President Viktor Yanukovich.
European Commission President Jose Manuel Barroso has also said he had no plans to visit Ukraine because of Tymoshenko who says she was jailed on Yanukovich's personal order and fears for her life.
The boycott of the informal summit - which was held in Poland last year and attended by 20 heads of states including U.S. President Barack Obama - risks embarrassing Kiev which says it wants eventually to join the European Union.
It could also be a precursor to an even more painful boycott of the European football championship which Ukraine is co-hosting with Poland next month and which Kiev has hoped would cement its position in the European mainstream.
In Poland, Tymoshenko's case emerged as a domestic political issue on Thursday as the main opposition party urged a boycott of matches held in Ukraine while President Bronislaw Komorowski and Prime Minister Donald Tusk spoke against such steps.
Yet, Tusk warned Ukraine its reputation would "suffer dramatically" if no humanitarian solution were found.
"I have left Ukraine's prime minister and president in no doubt that the (Tymoshenko) case ...is a test of credibility for the present Ukrainian authorities and that the reputation of Ukraine would suffer dramatically if it does not find a solution that we deem civilized before the European championship," Polish Prime Minister Donald Tusk told a news conference.
Tymoshenko, a former prime minister and Yanukovich's main political rival, was sentenced to seven years in prison last October for abuse of office after a trial that the West says was politically motivated.
She is now in a prison in the city of Kharkiv, one of the Euro-2012 venues. She has been on a hunger strike since April 20 in protest at what she said was an assault by prison guards, an allegation denied by the prison administration.
The prime minister of Lithuania, whose president Dalia Grybauskaite has still to decide on whether to attend the Yalta summit, called Tymoshenko's jailing a mistake on Thursday but said European leaders could use the meeting to discuss it.
"Such mistakes by Ukraine government are deplorable, they create many problems on Ukraine's path towards integration into the European Union," Andrius Kubilius told Ziniu radijas radio station in an interview on Thursday.
Matches between 16 European national teams will be held in Ukraine and Poland through June leading up to the final in Kiev on July 1.
WEST'S PLEAS IGNORED
Tymoshenko, 51, was one of the leaders of the 2004 Orange Revolution that derailed Yanukovich's first bid for presidency.
Her family says she is in poor health due to the hunger strike and chronic back pain but Tymoshenko refuses treatment, saying she does not trust state-appointed doctors.
Ukraine's foreign ministry and Yanukovich's office both declined to comment on the Yalta summit boycott on Thursday.
The foreign ministry last month said that five of the non-attending presidents had declined invitations to the Yalta meeting in advance for various reasons.
It has since, however, reacted angrily to news that German Chancellor Angela Merkel would not attend the soccer championship due to Tymoshenko's case, calling the move a return to Cold War tactics.
Ukrainian government and businessmen have invested billions of dollars in preparations for one of the main sports events on the continent - and the biggest one during Yanukovich's current term in office.
But the Kiev government has so far ignored Brussels' calls to free Tymoshenko and state prosecutors have brought to court fresh tax evasion charges against her which carry a sentence of up to 12 years.
The EU has warned Ukraine its members will not ratify milestone bilateral agreements on political association and free trade as long as Tymoshenko remains in prison.
"These (boycott moves) are the first steps towards international isolation, not for Ukraine but for Yanukovich personally," said Kiev-based political analyst Ihor Zhdanov.
"This will go on for as long as Tymoshenko remains in prison. But one must understand that Tymoshenko simply exemplifies all the problems with democracy in Ukraine."
Talks between Berlin and Kiev on arranging her treatment by German doctors have not yet resulted in any concrete agreement.
"Yulia Tymoshenko barely moves but does not plan to stop the hunger strike yet," Tymoshenko's political party Batkivshchyna (Fatherland) quoted her lawyer Serhiy Vlasenko sa saying in a statement on Thursday.
(Additional reporting by Angel Krasimirov in Sofia, Patrick Lannin in Vilnius and Zoran Radosavljevic in Zagreb; Writing by Olzhas Auyezov and Richard Balmforth Editing by Maria Golovnina)
Reuters
