Borisov.
Socialist party leader Sergey Stanishev said the discovery was "preparation for total falsification of the elections."
"350,000 ballots correspond to 10 per cent of the expected turnout tomorrow and ensure about 25 lawmakers' seats," he said on Saturday.
"This is a scandal unseen in Bulgaria so far," Stanishev fumed, directly accusing the conservative GERB party of Borisov and the ex-premier's campaign manager, former interior minister Tsvetan Tsvetanov.
The papers in Kostinbrod, 15 kilometres northwest of Sofia, exceeded the number that the privately owned printing company Multiprint was under contract to print.
"The ballots found on the night of May 10 were outside the ordered amounts," prosecutors said in a statement.
The government confirmed that it had indeed received the whole order of 8,343,000 ballots from Multiprint on May 8.
The owner of Multiprint denied any wrongdoing, saying that the extra papers were due to be thrown away. Prosecutors said however that they were packed up and ready to be despatched.