Ratings agency Moody's downgraded the debt of BNP Paribas, Societe Generale and Credit Agricole saying their creditworthiness was being hurt by the fragile operating environment for European banks.
Moody's cut its ratings on the long-term debt of BNP and Credit Agicole by one notch to Aa3, concluding reviews that began in June and were continued in September. Societe Generale's long-term debt was cut by one notch to A1.
The downgrades were driven by the increasing difficulties the banks were having in raising funding and the worsening economic outlook, Moody's said.
Late on Thursday Europe's banking watchdog, the European Banking Authority (EBA), said the region's banks must find 114.7 billion euros of extra capital, more than predicted two months ago, to make them strong enough to withstand the euro zone debt crisis and restore investor confidence.
But Moody's said its ratings did take into account the fact that all three French banks were likely to benefit from state support if the crisis deepened.
"Liquidity and funding conditions have deteriorated significantly," said Moody's, adding that the banks have historically relied on wholesale funding markets.
"The probability that the banks will face further funding pressures has risen in line with the worsening European debt crisis."
All three of the banks have undertaken programmes to sell assets to reduce their reliance on outside funding, but Moody's said that since many European banks were doing the same thing such asset sales might not generate as much money as the banks hoped.