It appears that Marcel Ospel, the group's new chairman, sacked Mr Arnold following a boardroom power struggle. The bank said that that Mr Arnold, 51, had resigned as president of the group executive board with immediate effect following a "number of differences of opinion".

“We regret the necessity to make this leadership change,” said Mr Ospel, "But we didn't want take any risks and we wanted the group's peace of mind." Shares in UBS fell almost 5 per cent to E82.35 in early trading.

Mr Arnold had pioneered the group's global share listing on the New York Stock Exchange and had overseen a sharp reduction in the group's risk profile. His moves to improve the quality and transparency of UBS's financial reporting had been welcomed by the investment community. He will be replaced by Peter Wuffli, 44, who has been responsible for turning round UBS's formerly troubled asset management operations.

"Ospel says that Arnold has done everything one would expect of a good CEO," said Markus Gisler, head of Moneycab, a financial website. "Arnold’s departure could be due to a number of strategic issues on which they disagree."

"They must have had other arguments to get rid of Arnold, because he was really appreciated," Gisler told swissinfo. "When you choose someone for the CEO’s job, you can’t suddenly decide you don’t like him shortly afterwards."

"I think Arnold's departure is a loss for UBS, but Peter Wuffli has been involved in banking strategy for a number of years," said Gisler. "He has one disadvantage though, because he does not have a very good reputation for public communication, and Arnold has done an excellent job communicating."