NEW YORK — Moody’s Investors Service said today it may lower the ratings of some of the world’s largest banks, as well as those of some securities firms, because their long-term prospects for profitability and growth are shrinking.

The ratings agency said that it is concerned that banks with significant capital market activities are dealing with challenges such as widening credit spreads, delicate funding conditions and increased regulatory requirements and restrictions.

The banks whose long-term ratings are under review for downgrade include Citigroup, Bank of America, Goldman Sachs, JPMorgan Chase, the Royal Bank of Canada and Morgan Stanley.

Moody’s also extended ongoing reviews for downgrades on 11 companies, which include Credit Suisse, Macquarie, Nomura, UBS, Barclays, BNP Paribas, Credit Agricole, Deutsche Bank, HSBC, Royal Bank of Scotland and Societe Generale.

The announcement of the potential downgrades comes just days after Moody’s said that it was cutting the ratings of Italy, Portugal and Spain because of uncertainty over the eurozone’s ability to enact reforms necessary to dig out of its debt crisis and Europe’s weakening economy.