After talks on resuming broadcasts on the ERT radio and television network collapsed, Democratic Left MPs voted to withdraw their ministers from the cabinet.
They have yet to decide whether to offer support in parliament to keep Greece’s international bailout on track.
The left-leaning party’s departure was blow to the conservative Mr Samaras, who is left with a three-seat majority, making it tougher to pass unpopular reforms demanded by foreign lenders and emboldening the hard-left opposition.
In a defiant address to Greeks in the early hours of yesterday, the prime minister said he was ready to press ahead.
“I want us to continue together as we started, but I will move on either way,” he said.
A spokesman said Mr Samaras had a sufficient majority “to lead the country out of the crisis”.
The row coincided with a new hitch in Greece’s European Union-International Monetary Fund bailout, with the discovery of a potential funding shortfall due to the reluctance of some eurozone central banks to roll over their holdings of Greek government bonds.
Mr Samaras’s conservative New Democracy party and its Socialist Pasok ally have 153 MPs in the 300-seat parliament, so they can continued for a while without the Democratic Left’s 14 MPs, but the outlook is more unstable.
“The government can’t last for long in its new shape,” John Loulis, a political analyst, said. “The horse-trading will begin, there will be more crises, they won’t be able to push reforms.”
Officials from all three parties ruled out snap elections, which would derail the bailout programme. “The country doesn’t need elections,” Democratic Left leader Fotis Kouvelis said, adding that his party would continue with its reform policy.
The decision to pull out was not unanimous and four party officials later called it a “suicidal” move. “This was a leap into the void,” they said in a statement.
An ongoing inspection visit to Greece by the EU and IMF needs to be completed as planned next month to avoid funding problems. That may require new savings measures to plug the gap.
At least two independent MPs have suggested they would back Mr Samaras’s government, which came to power a year ago in an uneasy pro-bailout coalition aimed at ensuring Greece stayed in the eurozone after nearly going bankrupt.
The coalition has bickered over a range of issues from austerity policies to immigration, and MPs from Mr Samaras’s party have accused Democratic Left of blocking public-sector reforms needed to secure bailout funds.
The latest crisis began ten days ago when Mr Samaras abruptly yanked the ERT radio and television station off the air and sacked its 2,600 workers, sparking an outcry from two groups of allies, unions and journalists.
Calling it a “sinful” and “wasteful” hotbed of political patronage, the prime minister said the move had been necessary to hit public sector lay-off targets set by Greece’s EU and IMF lenders.