Moody's Investors Service sent shockwaves through the global banking system as the ratings agency threatened to slash the credit scores of more than 100 banks in the wake of Europe's debt crisis.
The agency has put the ratings of 114 banks in Europe under review, as well as 17 investment banks. The move could hit 122 banks in total as nine of the investment banks are based in Europe.
All four of the UK's big banks - HSBC, Barclays, the state-backed Royal Bank of Scotland and Lloyds - face potential downgrades, affecting their ability to fund themselves in financial markets and potentially hampering an already weak recovery.
Michael Symonds, a bank analyst at Daiwa Capital Markets Europe, said: "The banks are getting it from all sides at the moment - the ratings are only going in one direction.
"All things being equal, a lower rating will make it more expensive for them to fund themselves and make funding sources scarcer, as some bond funds can only hold bonds of a certain rating."