UBS has had a difficult 2008
UBS has overhauled its management structure as it tries to recover from losses linked to the sub-prime crisis.
The Swiss bank, which has been the biggest European casualty of the credit crunch, is devolving many of the chairman's duties and responsibilities.
Four of UBS's 12 board members will step down in the firm's second shake-up since last October.
But the move failed to reassure investors, with shares falling as much as 6.3% to a 10-year low.
The Swiss bank's reputation for financial prudence has been recently damaged by the sub-prime crisis which has seen it make losses of more than $37bn (£18.6bn).
The firm made an $11bn loss in the first three months of 2008 and it is the process of cutting 5,500 jobs.
Analysts said that there has been a large amount of market speculation that UBS will announce more writedowns for the second quarter.
The recommendations for the restructuring came from the bank's governance committee - and included a clear separation of the responsibilities of the board of directors and executive management.
Key shareholder Olivant had been critical of the chairman's office - currently held by Peter Kurer, for having too much influence over executive management.
Mr Kurer said that "bringing UBS back to its leading position" was "the number one priority".
The move came as it emerged that US authorities had stepped up a probe into allegations that offshore accounts were set up at UBS to help wealthy clients avoid tax.
The Justice Department is seeking court approval for the Internal Revenue Service (IRS) to issue a civil summons against UBS to obtain key information.
"There is reason to suspect that..many US taxpayers used UBS's services to shelter their assets and, as a result, failed to file accurate tax returns," U.S. Attorney R. Alexander Acosta said in a statement to a Federal District Court in Florida.
A former UBS banker admitted last month to conspiring to help US clients avoid paying tax.
UBS said it took the issue "very seriously" and was co-operating fully.
UBS said it was "aware" of the summons request.
"UBS takes this matter very seriously and is working diligently with both Swiss and US government authorities, consistent with Swiss law and the legal frameworks for intergovernmental co-operation and assistance," it said in a statement.
Swiss officials held talks with their US counterparts about the allegations last week.
This followed the admission by Bradley Birkenfeld, a former director of UBS private banking arm, that he helped US clients avoid paying more than $7m in tax by hiding more $200m in an elaborate web of fraud.
According to the Justice Department, Mr Birkenfeld claimed more than $20bn was sitting in "undeclared" accounts at the Swiss bank controlled by US taxpayers.
Media reports have suggested that up to 20,000 US citizens may be involved.
The IRS said its probe would enable it to "detect wealthy individuals who don't pay their taxes as well as provide details about how advisers facilitate this abuse".