The Fitch credit ratings agency has said that if commercial lenders roll over their loans to Greece, it will deem the country to be in "default".As Greece awaits further bail-out money from the EU and International Monetary Fund, private investors are under pressure to extend their loans.
Last week, France and Germany reached a compromise over whether such investors should assume a greater burden, saying any such move should be "voluntary".
But few think their help is by choice.
Fitch's comments come as the Greek government is due to face a vote of confidence, a crucial first step towards gaining another 12bn-euro ($17bn; £10bn) loan from the EU and the IMF.
If the government survives the vote, Greece's parliament will be asked to back the latest spending cuts - worth 28bn euros - on 28 June.
'Junk' fears Fitch Ratings believes that any softening of terms by commercial banks would come only as a result of political pressure and therefore cannot be deemed "voluntary".
Categorising a borrower as "in default" will mean a further lowering of Greece's credit rating.
This is already deemed to be "junk", meaning that lenders are not expected to get back anything like the value of their original loan.
A further downgrade to default would mean a fire-sale of Greek loans, as certain investors would no longer be allowed to hold such risky assets.
Greece is trying to pass austerity measures through parliament in order to qualify for another slice of aid, worth 12bn euros ($17bn, £12bn) from the EU and IMF. The measures, which have cut benefits, public sector salaries and pensions, have sparked protests across the country.
In any case, that new money will still not be enough to keep Greece afloat long-term, and the institutions are planning to provide another bail-out, which could be worth more than another 100bn euros.
But amid political pressure from certain quarters, particularly the German government, this requires the willing contribution of private lenders.
Any contribution may prove worthless if it is viewed as a technical default.
Another agency, Standard & Poor's, has also warned that any attempt to restructure the country's debt would be considered a default.
The third leading agency, Moody's has a rating on Greece's debt that implies a 50% chance of a reneging on repayment within three to five years.