UBS Warburg, often regarded as a cautious player compared with its investment banking peers, has become one of the most aggressive proprietary traders in the market, according to analysts at rival Credit Suisse First Boston (CSFB).

"Contrary to some investors' perceptions, UBS is taking on more proprietary trading risk than the other European banks," CSFB's equity research analysts said in a recent report. They pointed out that the rise in UBS's fixed-income value-at-risk (VAR), a commonly used measure of how much proprietary trading a bank is engaged in, was second only to Citigroup last year. The Swiss bank's equity VAR also rose, and was behind only Goldman Sachs and BNP Paribas, according to CSFB. A source close to UBS said its risk levels in equity were at their lowest for two years. UBS posts first-quarter results next month.