UBS, the Swiss bank which has been laid low by losses in sub-prime mortgages, said it would replace four board directors in a corporate governance upheaval aimed at preventing too much power from being exercised by the bank's chairman.

The new governance model, which comes into force immediately, will separate the roles and responsibilities of the board of directors and the executive management. The changes follow criticism of the dominant role of the bank's former chairman Marcel Ospel who led the bank during a period in which it was forced to write off $37 billion (£18.6 billion) in assets accumulated during the US credit bubble.

UBS said that four directors, Stephan Haeringer who is currently non-independent vice chairman, Rolf Meyer, Peter Spuhler and Lawrence Weinbach, would leave the board in October. A new position of senior independent director has been created and the bank yesterday appointed Sergio Marchionne, the chief executive of Fiat and currently the Swiss bank's non-executive vice chairman to fill the new job.

The Swiss bank's internal shake-up was launched as America's Department of Justice piled further pressure on UBS in a widening investigation into the activities of alleged US tax evaders. In Miami, the Department of Justice asked a federal court to order UBS to hand over the names US citizens who may have hidden funds in UBS accounts.

The so-called "John Doe" summons, used to order the disclosure of information about unknown individuals would force the bank to identify Americans who held money in accounts that had not been disclosed to the US tax authorities. If the court complies with the request, it would be the first time that a non-US bank has been issued with such a summons and it could precipitate a major legal battle about America's jurisdiction over foreign financial institutions that have activities in America.

The US Internal Revenue Service believes that US clients hold some $20 billion (£10.04 billion) in undeclared assets in UBS accounts based on information supplied by Beradley Brikenfeld, a former UBS banker who admitted to helping his clients evade US taxes. UBS has said it would cooperate with the US authorities where its US clients have broken American law.

UBS said that its new corporate governance model would give the board of directors clear authority to set strategy by delegating the duties that were formerly allocated to the chairman's office to a number of board committees. These include new risk and strategy committees while the the remits of the Governance and Nominating Committees and the Human Resources and Compensation Committee have been expanded.

"Bringing UBS back to its leading position was the number one priority I committed to", said Peter Kurer, the UBS chairman who replaced Marcel Ospel. "We have made a big step forward with the clear separation of the duties between the Board and the Executive Management and the abolition of the chairman's office."

UBS said it had initiated a search process to narrow a list of candidates to replace the departing directors. It said the majority of the candidates would be expected to have substantial banking finance and risk backgrounds. An Extraordinary General Meeting will be held on October 2 to elect new board members.

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