Fitch cut its long-term rating on Greece to 'CC' from 'CCC' amid turmoil over the country's debt negotiations, that have led to fears that the country could make an exit from the euro zone.
Greece, which has received nearly 240 billion euros in two bailouts from the European Union and International Monetary Fund since 2010, is inching closer to a default on a 1.6 billion euro debt repayment to the IMF which falls due on Tuesday.
"GREECE CRISIS"
The ratings agency said the breakdown of the negotiations between the Greek government and its creditors has significantly increased the risk that the country will not be able to meet its debt obligations in the next few months.
The downgrade comes a day after S&P lowered its sovereign rating on the country and said that the probability of Greece exiting the euro zone was now about 50%.
"We now view a default on government debt held by private creditors as probable," Fitch said in a statement.
Fitch also lowered Greece's country ceiling to 'CCC' from 'B-' due to the imposition of capital controls and the risk of a permanent break from the euro zone.