The unit, which posted a 6 percent fall in second-quarter profit before tax, due in part to spending and provisions on legal cases, is "critical" to the Swiss bank's overall private banking strategy, Chief Executive Sergio Ermotti told analysts following its results.

"It's not so hard to see why this strong business, with its strategic and financial importance, looks attractive to our competitors, but it's worth even more to UBS and its shareholders, and that's why it's not for sale," Ermotti said.

(Reporting by Joshua Franklin and Oliver Hirt, writing by Katharina Bart, editing by Louise Heavens)