* Centre-right GERB demands election within six weeks
* Says campaigning on day before the vote hit support
* GERB's Borisov to propose minority government
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, 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 against low living standards, coruption 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.
Though the biggest party, GERB has little chance of forming a government because other groups refuse to work with it.
"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."
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.
Bulgaria urgently needs to negotiate EU funds for the next seven years, draft the 2014 budget and address popular anger.
ILLEGAL CAMPAIGNING
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 for the illegal ballot printing and said interim government officials had failed to exercise effective controls.
An alliance of Socialists and the MRF are one seat short of a majority and may seek support from individual MPs from GERB and the nationalist Attack. They will only have the chance to do so once GERB has tried and failed to form an administration.
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