In February, Moody’s announced it would review the ratings of 17 global investment banks and has already downgraded Macquarie and Nomura. In the U.S., the companies that are most likely to be affected by today’s action: Bank of America , Citigroup , Goldman Sachs , JPMorgan and Morgan Stanley .

Royal Bank of Canada and nine European banks, including Deutsche Bank , BNP Paribas and Credit Suisse are also on the list.

The current credit actions are part of a comprehensive review of the overall global banking system by Moody’s. In the middle of last month Moody’s downgraded Italian, Spanish, German and Austrian bank credit ratings. The U.S. banks with global capital markets capabilities have had an open dialogue with the ratings company, in an effort to soften the severity of the downgrades.

This afternoon’s anticipated announcement could affect the long-term debt ratings of the bank-holding companies of five of the biggest U.S. banks — only Wells Fargo is not on the list. Moody’s is also looking at the short-term debt of the five bank-holding companies and the main bank operating subsidiaries of all except JPMorgan.

The company that has the most to lose: Morgan Stanley. Moody’s is contemplating cutting Morgan Stanley’s senior long term debt rating three notches to Baa2, or just two notches above “junk”. That could put its credit rating on a level with Bank of America’s and Citigroup’s — if its debt is downgraded as expected.

In an interview with CNBC in April, Morgan Stanley Chairman and CEO, James Gorman said the downgrade could affect about 8 percent of the firm’s derivatives contracts. Downgrades of its senior debt could cost Morgan Stanley between $868 million and $7.2 billion in additional collateral and termination payments on derivatives contracts, according to SEC filings.

As for the other banks, dropping its senior debt rating to Baa2 would mean a one notch downgrade for Bank of America, and could mean the bank has to put up an additional $2.7 billion in collateral. The Moody’s downgrade would cost Citigroup an additional $1.1 billion in collateral.

If Goldman is downgraded two notches, as expected, it will end up with an A3 long term debt rating that would cost $2.2 billion in added collateral. JPMorgan Chase is expected to get a two notch downgrade to A2, costing the bank $4.7 billion in additional collateral and termination payments.

Correction: An earlier version of this story said that a Moody's downgrade would cost Citigroup an additional $4.7 billion in collateral. The correct amount is $1.1 billion.

- Written by CNBC's Mary Thompson and Margaret Popper