Moody's Investors Service has downgraded the sovereign credit rating of Greece to the lowest level, from 'Ca' to 'C', the rating agency said.

The agency said on its website that the decision "was prompted by the recently announced debt exchange proposals for Greece, which imply expected losses to investors in excess of 70%, which is consistent with Moody's criteria for a C rating."

Two weeks ago euro zone officials agreed to a second EUR 130 B bailout aid package for Greece and called for private investors to waive 53.5% of their principal in a bond swap to cut Greece's massive EUR 360 B debt by EUR 107 B.

Last week international ratings agency Standard & Poor's downgraded on Tuesday Greece's long-term sovereign credit rating from 'CC' to 'selective default.'

Moody's lowered Greece's sovereign rating to C from Ca, arguing that the risk of default remains high even a bond-swap deal with banks and other private investors, due to be completed this month, is successful.

It said it would "re-assess the credit risk profile'' after Greece issues the new bonds.

The swap deal aims to cut euro107 billion (4 billion) from the country's debt, and would see private investors lose more than half the face value of their Greek bonds in exchange for new ones issued with more favorable repayment terms for the crisis-hit country.

The exchange is an integral part of a second bailout package for Greece by other eurozone countries and the International Monetary Fund.

"Looking ahead, the EU program and proposed debt exchanges will reduce Greece's debt burden, but the risk of a default even after the debt exchange has been completed remains high,'' Moody's said.

"Moody's believes that Greece will still face medium-term solvency challenges: its stock of debt will still be well in excess of 100 percent of gross domestic product for many years, the country is unlikely to be able to access the private market once the second assistance package runs out, and its planned fiscal and economic reforms will still face very significant implementation risks.''