UBS is expected to announce today the departure of its investment bank head as it warns that it has written off billions of dollars on fixed-income assets, making the Swiss group the biggest casualty so far of the turmoil in global financial markets.

According to people familiar with the matter, the bank is expected to say it has written down its fixed-income portfolio by more than SFr3bn ($1.3bn), triggering a third-quarter loss of at least SFr600m.