Ratings agency Fitch has downgraded Greece's credit rating from CCC to C.
Greece on Tuesday agreed to further austerity measures in exchange for a huge bailout loan from the European Union and the International Monetary Fund.
Parliament now has a week to approve 3.3 billion euros in associated spending cuts.
The latest deal halves the face value of Greek government bonds, obliging private creditors to write off $US140 billion of Greek debt.
Greece also has to reduce its debt from 160% of GDP to 120.5% by 2020 and a constitutional change will give priority to debt repayments over the funding of government services
Eurozone leaders said the deal had saved Greece from going bankrupt.
