Standard & Poor’s plans to update its credit ratings for the world’s 30 biggest banks within three weeks and may well mete out a few downgrades in the process, possibly surprising battered global bond markets.

Among the institutions that could be downgraded are Bank of America Corp, Citigroup Inc. and Morgan Stanley, said Baylor Lancaster, an analyst at CreditSights Inc.

Spokesmen for the three banks declined to comment.

Some European banks could also be affected. On Nov. 9, S&P downgraded its scores for the health of the banking industries in a number of countries, including Denmark, Sweden, Finland and the Netherlands.

The updates in ratings are part of a major overhaul of S&P’s methods for scoring the creditworthiness of some 750 banking groups.