Fitch rating agency said it has downgraded the four top Greek banks to the lowest investment status.
It warned that the banks' outlook was negative and comes as a new blow to the debt-stricken Greek economy.
Fitch named the banks as National Bank of Greece (NBG), Alpha Bank (Alpha), Efg Eurobank Ergasias (Eurobank) and Piraeus Bank, saying it had downgraded by one notch their long-term risk of defaulting as issuers of debt to BBB and their short-term ratings to F3.
Fitch said the decisions reflected its view that expected budgetary cutbacks in Greece would put further pressure on the banks' already weakening assets.
In December, the agency had already downgraded these banks and one other by a notch as well as downgrading Greek sovereign debt.
The latest downgrading blow to a big slice of the Greek financial sector came the the day before the country was set to be hit by a nationwide strike.
The action is to protest ay measures by the Socialist government to cut state spending and pay and to increase taxes, under a crisis programme to reduce overspending.
Fitch said that the measures needed to rectify Greek public finances 'will have a significant effect on the real economy, affecting loan demand and putting additional pressure on asset quality'. That could lead to higher charges for credit and eventually weaken profitability.
Fitch, which has now downgraded the four banks to just one notch above non-investment grade status, sometimes referred to as the junk bond zone, said that the uncertainty over Greek public finances had already affected the possibilities for Greek banks to access funds on money markets 'at reasonable prices'. This meant they were relying to some degree on European Central Bank funding.