Fitch rating agency said on Friday it had downgraded one notch the notation for the five leading Greek banks to "B-" because of strains on their cash positions and assets.
The rating agency issued its new notation only hours before the publication of European Union stress tests on the ability of banks to withstand financial shocks.
Greek banks are receiving massive lifeline support from the European Central Bank, but Fitch said that this, coupled with a flight of capital abroad, left the banks with little room for manoeuvre.
But Fitch said the deep crisis in the Greek economy had led it to downgrade the credit standing of the banks in view of strains to their liquidity, on the effects of consolidating their accounts, and the assets in their balance sheets.
The banks affected are: Banque Nationale de Grece (BNG), Eurobank, Alpha, Piraeus and agriculture bank Ate.
These banks are on the list of 91 banks in 21 European countries covered by the stress tests to be published by the EU banking regulator EBA late on Friday.
Fitch also noted that the banks, holders of large amounts of Greek sovereign debt, were exposed directly to the Greek debt crisis.
A Greek banking source, who declined to be named, said that Greek banks hold Greek debt worth 48 billion euros ($68.0 billion), about the same amount held by the ECB which has bought government bonds from the financial sector under a special relief programme.
The downgrading follows a decision by Fitch to downgrade the notation of Greek sovereign debt to junk status on Wednesday, becoming the third top agency to do so.
When the last stress tests were carried out in July 2010, the only Greek bank to fail was Ate.
The ECB has warned that if a second rescue for Greece by the European Union involves a role for the private sector, which would probably trigger a default rating for Greece, it would probably cease funding the Greek banking system.