UPDATE 3-Bulgaria's leading party demands vote rerun after "violation"
* Centre-right GERB demands election within six weeks
* Says campaigning on day before the vote hit support
* Political vacuum further undermines EU's poorest country
By Tsvetelia Tsolova
SOFIA, May 16 (Reuters) - Bulgaria's GERB party is demanding a rerun of Sunday's election, blaming illegal campaigning for its failure to win more support and prolonging a political vacuum that would further undermine the European Union's poorest economy.
Centre-right GERB, which won 97 of 240 parliamentary seats and cannot form a majority coalition, was swept from office in February by street protests over living standards, corruption and organised crime.
Leader Boiko Borisov said on Thursday he will propose a minority government, even though it will not be voted in by parliament, if the constitutional court has not ruled on GERB's complaint before the new assembly is convened.
"In the configuration in which the four parties are at the moment, a government cannot be formed," Borisov said. "The new vote can be held in the next month, month and a half. Nothing worse can happen, from where we are now."
Bulgaria urgently needs a new government to negotiate EU funds for the next seven years, draft the 2014 budget and address popular anger.
A constitutional court ruling could take up to two months, legal experts said, during which time parliament could be convened and a new government voted in. The electoral commission and chief prosecutor said they saw no violations of the law.
"I believe there will be no legal consequences (of GERB's demand), but the political ones indicate a drive towards chaos, for instability in the country," Sergei Stanishev, leader of the Socialists, GERB's main rivals.
"They want to remain in power at all costs in order to cover the traces of their outrageous actions," he said.
President Rosen Plevneliev has appealed to political parties to hammer out a coalition, saying Bulgaria did not need a new election. He hopes to convene parliament by the end of May.
Smaller parties refuse to work with GERB but are struggling to cement their own alliances.
GERB says support was affected by an announcement from state prosecutors on Saturday, when campaigning is banned before voting, that illegal ballots were found at a printing shop owned by one of its councillors.
Two parties - the Socialists and ethnic Turkish MRF - held media conferences afterwards. Borisov has denied any wrongdoing or involvement with the ballots.
"Today or tomorrow GERB will appeal to the constitutional court and seek the cancellation of the election," said Borisov, a former bodyguard to dictator Todor Zhivkov. "Our reason is the gross violation of the law in the day before the vote."
Borisov said, in an interview with Trud newspaper on Wednesday, that the prosecutors' announcement knocked 5-6 percent off GERB's results.
Chief Prosecutor Sotir Tsatsarov said his office had not accused any political party of the illegal ballot printing and said interim government officials had failed to exercise effective controls.
An alliance of Socialists and the MRF - which said GERB's comments were an admission it had lost the election - are one seat short of a majority and may seek support from the nationalist Attack, or individual MPs from GERB and Attack.
But they will only have the chance to do so once GERB has tried and failed to form an administration.
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