More stress on its balance sheets is just about the last thing that the banking sector needs. The subprime mortgage crisis has already battered banks, leading to huge losses, scrambles for funding and free-falling banking shares. The S&P index of financial stocks has lost more than 30 percent so far this year. At its worst, the index plunged around 55 percent between a high in May last year and a low in June this year.

Now, after a brief respite, comes more bad news. First, hedge funds still seem to be wedded to betting on further losses. Laurence Fletcher, who writes about hedge funds here at Reuters, notes that more than 6 percent of British banks’ equity is on loan to short sellers.

More worrying yet for banks, however, may be their exposure to embattled Fannie Mae and Freddie Mac. In a report, Societe Generale economists estimate that U.S. commercial banks hold about $1 trillion in Fannie and Freddie debt. That amounts to a whopping 9 percent of the commercial banks’ balance sheets.

Then again, maybe the danger to the banks will simply add pressure on the U.S. government for make sure Fannie and Freddie don’t fail.