Standard & Poor's has warned that current proposals for restructuring Greece's debt would effectively constitute a default.
French banks, which hold some of the biggest exposures to Greek government debt, want to allow the country to extend the maturity of its bonds, which S&P said could be defined as a "selective default".
The euro fell against the dollar, losing around half a cent to $1.4513, after S&P released its analysis.
German and French banks have already agreed in principle to roll over loans to Greece in order to give the country more time to repay its debts.
This could involve effectively reinvesting the proceeds of maturing Greek debt into newly-issued bonds.
Standard & 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.
Under the Fédération Bancaire Française (FBF) plan, banks would invest some €30bn of maturing Greek debt in new bonds issued by Athens, which would not mature for up to 30 years. These securities would have an interest rate linked to Greece's GDP, and their sale would be restricted. The proposal won support last week from Germany, which is keen for private creditors to share the cost of a new rescue package worth an estimated €110bn (£100bn).
S&P, though, has concluded that this plan must be treated as a debt restructuring because investors would receive less value than was promised when they bought their original securities, and because without the deal Greece would almost certainly be unable to service its debts.
Chaos could ripple through financial markets if the rating agencies rule that Greece has defaulted. Banks would have to slash the value of the Greek bonds they hold, and would probably not be able to use them as collateral with the European Central Bank (ECB). There are also fears that Portugal and Ireland might also see their credit rating cut.