These pressures are unlikely to ease over the next 12-18 months, Moody's said, "and there is a high likelihood that capital controls and a deposit freeze will be imposed."
The credit ratings agency said the negative outlook also took into account the downward revision of Moody's 2015-16 GDP (gross domestic product) forecasts for the country, owing to political developments since the elections a few months ago which have led to a standstill in economic activity.
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Moody's now expects the country's economy to expand by just 0.5 percent this year – as does the European Commission, which earlier this month slashed its forecast from a previous estimate of 2.5 percent.
"The economic setback, coupled with Greek banks' higher funding costs and subdued revenue generation, will delay the recovery of pre-provision profitability in 2015, while banks will face rising problem loans and eroding capital buffers," Moody's said in the report.
"We expect dependence on central bank funding to remain high as depositor confidence remains compromised by ongoing uncertainty regarding Greece's support programme and related fears of potential capital controls and an exit from the euro area."