Ratings agency Standard & Poor's on Tuesday said it had downgraded the ratings of major US banks, including Citigroup, Goldman Sachs, Wells Fargo, JPMorgan Chase, Morgan Stanley and Bank of America.

The firm said it had revisited ratings on 37 of the world's largest banks, part of a process of "applying its new ratings criteria for banks."

S&P announced revised criteria to simplify ratings last week, after more than a year of study.

The changes reassess risk associated with investment banking, how banks are funded, how liquid banks are and their capital standards.

Part of the rationale for the move was to allow more room for banks to improve their ratings, if their finances were deemed more secure.

The ratings agencies were heavily criticized for failing to spot the 2008 financial crisis as it approached.

Although Tuesday's move was broadly expected, it sent shares in each firm down in after-hours trade.

Goldman Sachs shares fell 1.0 percent in after-hours trade, Bank of America and Citi were down 0.6 percent, Wells Fargo was down 0.8 percent and JPMorgan was down 0.4 percent.

Morgan Stanley was down 1.7 percent.

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