UBS announced the departure of its investment bank head as it warned that it has written off billions of dollars on fixed-income assets, making the Swiss banking group the biggest casualty so far of the turmoil in the global financial markets.

The bank said it had written down its fixed-income portfolio by SFr4bn ($3.4bn), mainly because of losses on US subprime mortgage-related securities, triggering a third-quarter loss of SFr600m-SFr800m.