Europe's banks, sitting on $1.19 trillion of debt to Spain, Portugal, Italy and Ireland, are facing a wave of losses if Greece abandons the euro.

While lenders have increased capital buffers, written down Greek bonds and used central-bank loans to help refinance units in southern Europe, they remain vulnerable to the contagion that might follow a withdrawal, investors say. Even with more than two years of preparation, banks still are at risk of deposit flight and rising defaults in other indebted euro nations.

“A Greek exit would be a Pandora's box,” said Jacques-Pascal Porta, who helps manage $570 million at Ofi Gestion Privee in Paris, including shares in Deutsche Bank AG and BNP Paribas SA. “It's a disaster that would leave the door open to other disasters. The euro's credibility will be weakened, and it would set a precedent: Why couldn't an exit happen for Spain, for Italy, and even for France?”