It has dropped long and short-term sovereign credit ratings to CCC+/C from B-/B and says its outlook is negative.
Markets use sovereign ratings to work out the interest rate at which investors should lend to a country.
Official figures on Wednesday also showed Greece’s deficit last year was higher than government forecasts.
The budget deficit – the difference between its revenue and spending – was 3.5% of GDP, compared with the prediction of 0.8%. The worsening finances of the government could see Greece’s creditors pushing for further austerity, experts said.
Meanwhile, German Finance Minister Wolfgang Schaeuble warned that an agreement between Athens and its creditors was unlikely to happen any time soon.
“Until now, we don’t have a solution, and I don’t expect to get a solution in the next week,” he said. Greece is meeting with its creditors in the Latvian capital, Riga, on 24 April.
The Syriza government is negotiating with the International Monetary Fund (IMF) and its eurozone partners in an attempt to lessen the burden of its debt repayments. A €750m ($800m; £540m) payment is due on 12 May, but the government will struggle to make it.
Athens faces a “choice between paying the IMF or paying the wages and pensions of its employees”, Raoul Ruparel, head of economic research at Open Europe, told the BBC.
“For a radical left-wing government such as Syriza, that is a very poisonous choice.”