Standard & Poor's downgraded Greece's bonds to junk status on Tuesday, a move which if replicated by fellow rating agencies would make the country's bonds ineligible under the European Central Bank's current collateral rules.

Standard & Poors downgraded Greece's short-term and long-term debt from BB+ to BBB+ after the rating agency updated its assessment of the political, economic, and budgetary challenges in Greece, as the country attempts to tackle its growing debt burden amid political unrest