The moderate Democratic Left party in Greece says it will not join pro-bailout parties in a coalition without the more radical far left Syriza.
The Greek president has called the four main parties, including the centre-right New Democracy and the Socialist Pasok, to try to form an emergency government to avoid new elections.
But Syriza said it would not attend because it could not back any coalition which supported austerity.
Nikos Pappas, Director of the office of Syriza's leader called the austerity programme "fundamentally wrong" and told John Humphrys that the party "will not accept" the measures.
"A default within the euro is possible," he said, adding that the euro system "is fundamentally wrong".
Elena Panaritis, an economist and MP for Pasok, told the Today programme that the Greek people "have confused reforms with austerity" and that Pasok's aim is to work towards providing a "strong front" that is best for Greece.
Greece now having "conversations we should have had two and a half years ago," she said.
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