The rating agency Standard & Poor's on Monday downgraded Greece's sovereign debt rating to CCC- from CCC. The outlook is negative.

The rating agency said that the likelihood is 50% that Greece eventually exits the Eurozone. Standard & Poor's forecasts a default within six months.

"In our view, the Greek government's decision to hold a national referendum on official creditors' loan proposals indicates that Prime Minister Alexis Tsipras will prioritize domestic politics over the country's financial and economic stability, commercial debt service, and membership of the Eurozone," the agency said.