Standard & Poor's Ratings Services downgraded more than a dozen large banks, including the six biggest US financial institutions, in the latest shot across the financial sector's bow from a major credit-rating firm.

S&P downgraded 15 big banks to reflect new rating methods the firm has been putting in place over the past year. The new guidelines sharpen the focus on how banks would hold up under market and economic stress, and on the likelihood of governments providing extraordinary support to troubled institutions.