The banks were required in an agreement with international creditors to guarantee 10 percent of their shares would be bought by private parties. But they could not make that guarantee, officials of the banks said.
The merger would have created the largest bank in Greece, with $234 billion in assets, close to the size of the country's gross domestic product, which was $247 billion in 2012.
The potential size of the new bank was a concern among Europe's financial leaders, who have already assembled a $170 billion international aid package for Greece.
Leaders were concerned the merger would create a bank that was "too big to fail," The New York Times reported Monday.
Talks on the merger could resume next month, the Times said, after a plan to recapitalize the National Bank of Greece, Eurobank, Alpha Bank and Piraeus Bank is completed.