At that time, said Antonis Samaras of the New Democracy party, an interim government would be formed in a bid to safeguard a new European bailout agreement and the next installment of rescue loans to the cash-strapped Mediterranean country.
Samaras' fiery demand came as Papandreou moved to scrap a controversial plan to hold a referendum on the bailout plan.
"I made my overture," Samaras told lawmakers, saying he had won the support of rival conservatives. “But I never accepted a deal to co-govern.”
Samaras said he proposed the creation of an interim government and suggested elections follow six weeks later.
Papandreou’s proposed referendum caused an uproar in Greece and abroad, where fears were mounting that the Mediterranean country would abandon the euro currency it shares with 17 other nations, and trigger a global financial crisis.
Aides to Papandreou said the prime minister was scheduled to speak with his conservative rival later Thursday, but the fate of a unity government remained in question.
-- Anthee Carassava
Photo: Greece's main conservative opposition leader, Antonis Samaras, speaks at the Greek parliament on Thursday during a session debating the issue of a confidence vote in the government. Credit: Louisa Gouliamaki / Agence France-Presse/Getty Images