"We base today's rating actions on our opinion of the increasing risks that Europe's large banking groups active in investment banking face as regulators and uncertain market conditions continue to make operating in the industry more difficult," the bank said in a statement.
The outlook on all three banks is now stable, meaning further cuts to their credit ratings are unlikely in the near term.
The downgrades come amid a shifting regulatory landscape for banks, changes that are forcing some of the largest firms to make significant changes to their operations.
In addition, large banks are now working to raise funds required to meet new capital requirements, at a time when market volatility and a dour economic environment could impact earnings.
"We consider that these banks' debtholders face heightened credit risk, owing to the industry's tighter regulation, fragile global markets, stagnant European economies, and rising litigation risk, stemming from the financial crisis," S&P said.,
