Standard and Poor's has warned that current proposals for restructuring Greece's debt would effectively constitute a default.
The ratings agency says plans for the private sector to roll over debts could trigger a default under its ratings criteria , the BBC reports.
The Greek parliament last week passed tough austerity measures to secure further financial aid, despite a two-day nationwide strike and violent protests in Athens against the measures.
German and French banks have already agreed in principle to roll over loans to Greece, which could involve effectively reinvesting the proceeds of maturing Greek debt into newly-issued bonds.
Standard and Poor's said that, depending on the circumstances, it viewed certain types of debt exchanges and similar restructurings as equivalent to a payment default.
The options laid out so far for restructuring Greek debt would constitute such a default, it said.
Last month, Standard and Poor's downgraded Greece from B to CCC.