Greek PM Papandreou faces party revolt over bailout
One MP has defected from Pasok, cutting Mr Papandreou's parliamentary majority to two seats - 152 out of 300 - ahead of a confidence vote on Friday.
The opposition has called for early elections, saying the referendum jeopardises Greece's EU membership.
European markets have fallen sharply.
Last week, eurozone leaders agreed on a 100bn-euro loan (£86bn; $140bn) to Athens and a 50% debt write-off.
In return, Greece must make deep cuts in public spending, slashing pensions and wages and making thousands of civil servants redundant.'Blackmail'
Earlier on Tuesday, Pasok MP Milena Apostolaki quit the parliamentary group but said she would keep her seat to fight the referendum as an independent.
End Quote French-German joint statement
France and Germany are convinced that this agreement will allow Greece to return to sustainable growth”
"I have an obligation to resist this erroneous political choice that divides the nation," she said.
Shortly afterwards, another MP from Mr Papandreou's centre-left Pasok party, Vasso Papandreou (no relation) called for a government of national unity, to be followed by snap elections.
"I call on the president to convene the council of political leaders with the goal of forming a government of national salvation in view of safeguarding the EU package agreed on 27 October, and then to immediately hold elections," said Ms Papandreou, who chairs the parliamentary committee on economic affairs........................................................ more