Greece: Voluntary bank help would be a default
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 came hours before the Greek government won vote of confidence: a crucial step towards gaining another 12bn euro ($17bn; £10bn) loan from the EU and the IMF.
Greece's parliament will now be asked to back the government's latest spending cuts, which are worth 28bn euros, on 28 June.
Spending cut-backs have already hit benefits, public sector salaries and pensions, sparking protests across the country.
That will not be enough to keep the country going, and another giant support package is being pondered, possibly worth an even greater sum than the original 112bn euros.
This time, commercial lenders such as banks, insurance companies and pension funds are being asked to contribute by voluntarily rolling over their existing loans.
"Voluntary" help by commercial lenders would mean a loan is paid back on time, but the same amount is immediately lent out again on the same terms.
In the case of Greece, this effectively means lending on easier terms because, given the current state of the Greek economy, lenders would not be expected to offer the same terms as they did a few years ago.
Fitch Ratings believes that any softening by commercial lenders would come only as a result of political pressure and therefore cannot be deemed voluntary.
BBC business editor Robert Peston said: "It is simply taken as fact everywhere but the conclave of eurozone finance ministers that Greece has borrowed perhaps twice what it can afford.
"There is therefore no banker, or pension fund manager or insurance executive anywhere in the world who, left to his or her own devices, would willingly lend more money to Greece."
That suggests any lending will only happen under duress and therefore come under Fitch's default category.
Categorising Greece as "in default", or unable to repay its debts, could trigger another financial meltdown and credit squeeze, as lenders, including banks, pension funds, and insurance companies, accept they will never get back the amount they lent.
Greek debt is already deemed to be "junk", but a further downgrade to default would also mean a fire-sale of Greek loans, as certain investors would no longer be allowed to hold such risky assets.
Neil MacKinnon, head of global strategy at VTB Capital, said it was becoming inevitable that Greece would fail to pay its debts. "Throwing more money at the problem might buy time but it doesn't solve the underlying problems and obviously those problems are very serious, and the prospect of debt default for Greece is now looking a very high probability."
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.