UBS, one of the biggest casualties of the US subprime crisis, yesterday suffered a further $4 billion (€2.7 billion) writedown on its mortgage-related securities, leading it to predict a SFr4.4 billion 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.

The news will increase pressure on its chairman, Marcel Ospel, who is facing mounting opposition from some shareholders, and who has stressed his wish to remain at the helm until the bank's problems are solved.

UBS said its 2007 loss, signalled last month with the initial disclosure of $10 billion in fourth-quarter writedowns, stemmed from additional losses on its portfolios in recent weeks, and very weak trading in fixed income, currencies and commodities.

The latest writedowns take to $18 billion the total UBS has lost in 2007 on US mortgage-related securities, including $4 billion of writedowns reported in the third quarter.

Some analysts fear further losses may follow, as UBS's markdowns on its most troubled securities are still smaller than the level of other big market participants, such as Merrill Lynch.

"The increased writedowns may well still not be enough," noted Jeremy Sigee, analyst at Citigroup. UBS shares fell 1.6 per cent to SFr46.06.

The bank, once renowned for caution and rigorous controls, shocked investors last year after admitting it held more than $40 billion in subprime-related paper via positions taken by Dillon Read Capital Management, its now closed internal hedge fund, and its conventional fixed-income trading activities.