Bulgaria's centre-right GERB, which won most parliamentary seats in Sunday's vote, wants the election result cancelled because of a "violation", its leader Boiko Borisov said on Thursday.
GERB won 97 seats in the 240-member parliament and says support was hit 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.
The party, which resigned from government after February street protests against low living standards in the European Union's poorest country, has little chance of forming a government because other groups refuse to work with it.
"Today or tomorrow GERB will appeal to the constitutional court and seek the cancellation of the election," said Borisov, who wants a re-run of the election. "Our reason is the gross violation of the law in the day before the vote."
If the constitutional court does not rule before the new parliament is convened, which the president hopes to do by the end of May, GERB will try and form a minority government even though it knows it will not be voted in by parliament, Borisov said.
An alliance of Socialists and the ethnic Turkish MRF party are one seat short of a majority and may seek support from individual GERB MPs and nationalist Attack. They will only have the chance to do so once GERB has tried and failed to form an administration.