UBS, one of the biggest casualties of the US subprime crisis, on Wednesday suffered a further $4bn writedown on its mortgage-related securities, leading it to predict a SFr4.4bn ($4bn) group net loss for 2007.

The surprise announcement, a fortnight before the Swiss bank’s full results, compares with a SFr12.3bn net profit in 2006 and shows the severity of UBS’s exposure to US residential securities.