Greek Prime Minister George Papandreou has lost the support of three of his MPs ahead of a crucial confidence motion in his government on Friday.
The Greek cabinet is meeting in emergency session at 10:00 GMT (12:00 local) after governing Pasok party MPs withdrew their support for a referendum on the eurozone bailout for Greece.
Pasok holds 152 of 300 seats, implying Mr Papandreou could face defeat.
Several government ministers have criticised the idea of a referendum.
The row threatened to overshadow a meeting of the G20 in Cannes, where heads of the group of leading industrialised nations are to discuss the eurozone debt crisis at a two-day summit.
On Monday, Mr Papandreou shocked European leaders and financial markets by calling a vote on the eurozone bailout plan.
Early on Thursday, Finance Minister Evangelos Venizelos spoke out publicly against the idea of a referendum. He was followed by the deputy finance minister, the health minister and the development minister, says the BBC's Mark Lowen, in Athens.
In a statement early on Thursday, Mr Venizelos said Greece's membership of the euro could not be put in doubt. "If we want to protect the country we must, under conditions of national unity and political seriousness and consensus, implement without any delay the decision of 26 October. Now, as soon as possible," Mr Venizelos said.

Eurozone leaders had hoped to present a definitive action plan for Greece, including a move by wealthy emerging economies such as China to contribute to expanding the European Financial Stability Fund (EFSF).
On 26 October, eurozone countries agreed to give Greece a second bailout of 130bn euros (£111bn; $178bn). Private banks would also write off 50% of the Greek state debts they hold.
In return, Greece must enact austerity measures, cutting wages and salaries, and making thousands of civil servants redundant.
Mr Papandreou plans to put the bailout to a popular vote on 4 or 5 December. However, our correspondent says the idea of a referendum is now in doubt, as the collapse of the government would trigger early elections.
Several Pasok MPs have called instead for a parliamentary vote on whether to accept the bailout terms, a government of national unity or early elections.
Another has called on Mr Papandreou to resign.
The EU says it will not disburse rescue funds until after the referendum.
Our correspondent says Mr Venizelos' statement exposes a rift at the very heart of the Greek government.
One of Mr Venizelos' advisers told the BBC the minister did not approve of the referendum and does not consider it a priority.
On Wednesday, Mr Papandreou said that his shock decision to call a national vote on the bailout package was effectively a decision on the country remaining part of the euro bloc.
Mr Venizelos travelled to Cannes on Wednesday for meetings at the G20 summit of leading industrialised countries.
He issued his statement upon his return to Athens at 04:45 local time (06:45 BST).
Greece was due to receive the next tranche of funds from its first bailout later this month. However, the EU has said it will not transfer the 8bn euros (£6.88bn; $10.9bn) until after the referendum.
As a result, Greece may confront difficulties paying pensions and salaries.