Ratings agency Moody's will announce its downgrades of many of the world's biggest banks later on Thursday, Sky News reported.
Moody's has said it will release the ratings reviews, which include significant downgrades for many banks, by the end of June. A spokeswoman for Moody's declined to comment on the exact timing.
There has been speculation the downgrades were imminent for several days.
Funding costs for the banks are likely to rise after the downgrades, and their trading partners may ask for more collateral. Much of the impact could already be discounted, however, as Moody's signaled in February how much banks like Morgan Stanley, HSBC, Deutsche Bank and Goldman Sachs were likely to be cut.