(Reuters) - Greek Prime Minister George Papandreou will resign as soon as a deal for an interim coalition government is reached, perhaps as early as Sunday night, a senior member of his party said.

The main goal, duration and leader of such an administration must be agreed before Papandreou resigns, PASOK party lawmaker Telemachos Hitiris told Greek state television.

We only have to wait for the prime minister's announcements in the cabinet, he said. Everything must be done within the day, otherwise tomorrow it will be hell.

Papandreou has come under fire at home and abroad for his short-lived plan for a referendum on a Eurozone bailout deal. He survived a confidence vote in parliament on Saturday, but lawmakers from his party have called on him to quit.

Hitiris said elections should be held after Greece meets its obligations to Eurozone partners, pinning the timing to around January or February.