International ratings agency Standard & Poor's said Monday there was a one in three chance that Greece will leave the eurozone in the months following 17 June polls.
"We believe there is at least a one in three chance of Greece exiting the eurozone in the coming months," if, for example, it were to reject the austerity measures and reforms agreed to in exchange for a massive EU-IMF bailout, S&P said.
Turning its back on the deal could lead to "a consequent suspension of external financial support," it said.
"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."
Greece goes to the polls for the second time in six weeks on 17 June after an inconclusive vote on 6 May saw parties supporting the EU-IMF debt accord take a drubbing from voters hostile to endless austerity and higher taxes.
The 17 June vote has turned into a straight vote on the pact and the associated austerity measures, with the latest opinion polls showing those for and against in a tight race which could have untold consequences for the future of the eurozone.
In a summary of a special report, S&P noted that the impact on other weaker eurozone states of a Greek exit "would be less clear-cut.
"We believe that other sovereigns would be unlikely to follow any Greek exit, having witnessed the resulting economic hardships ... (while) in the meantime their European partners would provide additional support to discourage further departures," it said in a statement.
SP said European policymakers would be keen to demonstrate that "Greece is a special case.
"We would expect growing financial support and leniency in the face of slipping targets for other sovereigns embroiled in the debt crisis.
"Accordingly, we currently do not consider that a Greek withdrawal would automatically have any permanently negative consequences for other peripheral sovereigns' prospects of
continuing eurozone membership."
A Greek exit would also "likely strengthen the resolve of other countries receiving external support to pursue reforms and avoid the economic consequences of an exit."
Greece had to be bailed out by the EU and International Monetary Fund for a first time in May 2010 and then needed more help, resulting in the latest debt accord which took months of talks to reach.
The eurozone debt crisis snared Ireland and then Portugal in turn, causing turmoil on the financial markets amid speculation that weaker member states such as Spain or even Italy could need a bailout too.