Standard & Poor's warned that it could further downgrade Greece's credit rating in the month ahead, citing risks to the country's growth forecast and public support for its reform measures.
The agency said it was maintaining its current rating on Greece's long- and short-term debt as well as its negative outlook on the country, which is struggling to curb its debt and public deficit.
"In our view, a further downgrade of one or two notches is possible within a month," said S&P credit analyst Marko Mrsnik.
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A new downgrade could put Greece in a high-risk investment category.
Protests hit Greece
The report came as tens of thousands of people protested in Greek cities against austerity measures planned by the government, which wants to borrow around 54 billion euros (75 billion dollars) this year to meet its obligations.
Standard & Poor's in December joined two other international agencies, Fitch and Moody's, in lowering Greece's credit rating in the face of a public deficit that came to 12.7 percent last year, the highest in the eurozone.
The country's Socialist government, under heavy pressure from its eurozone partners, has since then pledged to freeze public wages and increase fuel taxes, among other measures, in a bid to shave the deficit to 8.7 percent of output this year.
Reducing 'public debt burden'
S&P described such moves as "a reasonable step toward reducing the public debt burden."
But it found that "downside risks" to Greece's growth prospects were likely to make necessary more stringent measures, "raising questions about the feasibility of the country's ambitious budgetary goals."
It described the growth assumptions underlying the government's plans for 2010 as "overly optimistic."
The agency also warned that if public backing for the austerity measures declined from its current level, implementation of the program could be jeopardised.
Despite the scale of Wednesday's strike and protests, polls suggest that more than six out of 10 Greeks support the government.
Standard & Poor's downgraded Greece's long-term debt rating to BBB-plus and its short-term rating to A-2 in December.
