Credit rating agency Standard and Poor's said on Monday that it sees "at least" a one-in-three chance of Greece exiting the eurozone in the coming months, an outcome which could lead the financially fragile country to another sovereign default.

"This could be brought about by Greece rejecting the reforms demanded by the troika — the European Commission, International Monetary Fund and European Central Bank — and a consequent suspension of external financial support," S&P said in a statement accompanying a new report.

"Such an outcome would, in our view, seriously damage Greece's economy and fiscal position in the medium term and most likely lead to another Greek sovereign default," it said.

But the impact of a Greek exit on other fiscally weak eurozone countries could be less clear cut, S&P said, with other sovereigns unlikely to follow Greece out of the monetary union.

"We believe that the hardships the Greek population would suffer were Greece to exit would dissuade any other member state from following suit," S&P said in the report.

The rating agency added that "it is our base-case assumption that a Greek exit by itself would not automatically trigger further downward sovereign rating actions elsewhere. Much will depend, however, on the European policy reaction following a Greek exit."

The S&P report came a day before finance chiefs of the Group of Seven leading industrialized powers are to hold emergency talks on the eurozone debt crisis in a sign of heightened global alarm about strains in the 17-member European currency area.

Global markets have been rattled by new signs of strain in the eurozone including political turmoil in Greece as austerity fatigue sets in among voters who are rejecting a push for yet more belt tightening.