Standard & Poor's Ratings Services lowered the long-term counterparty credit ratings of Barclays Bank PLC, Credit Suisse AG and Deutsche Bank AG to "A'' from "A-plus" on Tuesday.
The agency kept its "A/A-1" long and short-term ratings on Switzerland's UBS AG.
All remain at investment grade, and S&P said the outlooks on all four are stable.
"We see increasing risks for some large Europe-based banks operating in investment banking, as regulators and uncertain market conditions make operating in the industry more difficult," S&P said. In particular, these banks' debtholders face higher credit risks because of tighter regulation, fragile global markets, stagnant European economies, and rising risk of financial crisis-related lawsuits.
S&P said Barclays, Credit Suisse, Deutsche Bank, and UBS are "among the most exposed in Europe" to global regulatory changes.