Ratings agency Fitch cut Greece's credit rating Tuesday from "CCC" to "CC" -- meaning it is highly vulnerable to default -- following the breakdown of bailout talks and the Greek government's decision to call a referendum.

"The breakdown of the negotiations between the Greek government and its creditors has significantly increased the risk that Greece will not be able to honour its debt obligations in the coming months, including bonds held by the private sector," Fitch said.

"We now view a default on government debt held by private creditors as probable."