The board said in a statement that it regretted Mr. Grübel’s decision and that it had appointed a board member, Sergio P. Ermotti, to the chief executive position on an interim basis.

“Oswald Grübel feels that it is his duty to assume responsibility for the recent unauthorized trading incident,” the board chairman Kaspar Villiger said in the statement. “It is testimony to his uncompromising principles and integrity.”

A trader in the bank’s London office, Kweku M. Adoboli, 31, was arrested more than a week ago and charged with one count of fraud and two counts of false accounting dating to as early as October 2008. Mr. Adoboli, who worked at UBS Delta One desk, was accused of making billions of dollars in unauthorized investments in equity index futures and concealing them from internal risk controls.

Mr. Villiger also thanked Mr. Grübel “for everything he has done for UBS.” The bank added that the board was “deeply disappointed by the recent loss arising from unauthorized trading.”

The resignation comes about a week after Mr. Grübel told a Swiss newspaper that as chief executive he was responsible for the trading loss, but that he did not feel guilty. He also said he had not thought about resigning.

Mr. Grübel was brought out of retirement by Mr. Villiger to take over at UBS in 2009 after the bank suffered significant losses because of its exposure to the subprime mortgage market. During his tenure, Mr. Grübel managed to return UBS to profit by reversing client money outflows at its private banking business and by reducing costs.

The announcement Saturday came after a three-day board meeting in Singapore that was already scheduled before the unauthorized trades were revealed. The board was scheduled to discuss UBS’s future strategy especially with a focus on scaling back the investment banking business to make the entire bank more profitable.

The board said Saturday that it would stick to its integrated strategy, which means it would keep some investment banking operations. But the board asked the bank’s management to accelerate the planned changes to the investment banking unit, which are aimed at reducing costs, while ensuring the bank’s wealth management clients have access to enough products and services.

“We are committed to further expanding our already leading global wealth management franchise,” Mr. Villiger said in the statement. “The investment bank will continue to strengthen its alignment with UBS’s wealth management businesses, in addition to serving its corporate, sovereign and other institutional clients.”

Mr. Grübel’s resignation is the latest blow for UBS, which had struggled to recover from the giant losses it incurred during the credit crisis. He had enjoyed support both from the board and UBS staff and was seen as one of the few executives who could help the Swiss bank return to strength.

The bank had just started to recover from a damaging legal case in the United States involving its wealth management operation and clients’ tax payments, which had hurt its reputation and prompted customers to take their money elsewhere. Mr. Grübel went on a client charm offensive, hired client advisers and started a new advertising campaign to repair the banks reputation.

But UBS also struggled to retain talent especially in the investment banking operation as bankers felt management’s focus shifted toward the more successful wealth management unit, widely considered to be the crown jewels of the company. UBS also felt under pressure from Swiss regulators to shore up its capital thereby weighing on the bank’s profitability.

The revelation last week that unauthorized trades had resulted in a $2.3 billion loss might have pushed Mr. Grübel over the edge.

UBS had planned to scale back its fixed-income operation and other capital-intensive divisions to help make UBS more profitable before the trading loss was uncovered. But the scandal forced UBS to consider even greater changes and to reveal the plans before the investor meeting scheduled for Nov. 17.

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