Bulgaria is facing political deadlock after the Socialist Party insisted that it must lead a coalition government.
Mr Stanishev says Bulgaria must not waste time haggling
The Socialists (BSP) emerged on top in Saturday's elections, winning 31% of the vote, ahead of the ruling centrists led by ex-king Simeon Saxe-Coburg.
Mr Saxe-Coburg, the outgoing prime minister, has refused to join a Socialist-dominated government.
He said "a lot more time" would be needed for coalition negotiations in Bulgaria, which plans to join the EU.
BSP leader Sergei Stanishev said any future coalition "should be led by the party which won most votes in the elections".
Bulgaria is due to join the EU in 2007 - but its accession might be delayed if the EU is not satisfied with its progress in implementing reforms.
Senior officials in the ex-king's Simeon II National Movement (NMS) still expect to share power with the BSP - the former communists.
"It is quite likely," said Milen Velchev, the outgoing finance minister.
The BBC's Nick Thorpe in Sofia says the BSP now likes to be defined as "centre-left" and the conservative parties as "centre-right", leaving the NMS with a monopoly on the "pure centre" of Bulgarian politics.
The NMS came second with just under 20% of the vote and third with 12.7% was the Movement for Rights and Freedoms (MRF), a mainly ethnic-Turkish liberal party.
A radical nationalist group called Ataka won 8% - enough to enter parliament. It challenges Bulgaria's tentative efforts to integrate its Turkish and Roma (Gypsy) minorities.