It was reviewing the long-term ratings and standalone credit assessments of a range of banks, Moody's added. Markets were unaffected by the Moody's announcement.
"Capital markets firms are confronting evolving challenges, such as more fragile funding conditions, wider credit spreads, increased regulatory burdens and more difficult operating conditions," the ratings agency said in a statement.
It said among 17 banks and securities firms with global capital markets operations, it might cut the long-term credit rating of UBS, Credit Suisse and Morgan Stanley by as much as three notches following the review. It said the guidance was indicative.
Among the banks that might be downgraded by two notches are Barclays, BNP Paribas, Credit Agricole, Deutsche Bank, HSBC Holdings, and Goldman Sachs.
Bank of America and Nomura were included in those that might be downgraded by one notch.