The chief executive of UBS was ousted today as the flames of exposure to the global credit crisis, accusations of helping rich Americans avoid tax and links to the Madoff scandal engulf the giant Swiss bank.

Marcel Rohner, 43, who had been in the job for the past 21 months, leaves with immediate effect.

He is to be replaced by 40-year Credit Suisse veteran Oswald Grübel, 65, who was chief executive of UBS's Swiss investment banking rival from 2003 to 2007. Grübel took early retirement from Credit Suisse in 2002 but the bank brought him back.

UBS admitted that Rohner's departure is an attempt to restore its credibility. It added: “This represents a further step to restore stakeholder confidence and to pave the way back to success.”

Chairman Peter Kurer said: “With his indisputable leadership qualities combined with his extensive expertise, Oswald Grübel brings the ideal skill set to recreate value.

“He will also be adept in balancing our focus on prudent risk-taking and client confidence and our goal to position UBS for future success.”

Kurer claimed Rohner told the board in early January that he wanted to “retire” after he had finished restructuring UBS's stricken investment banking and wealth management arms. The move led to the controversial cull of 2000 bankers.

However, there was no indication Rohner was thinking of quitting UBS when he told reporters earlier this month that he continued to have the backing of the board.

Under his watch, UBS reported losses in the fourth quarter of 2008 of Swfr8.1 billion (£4.9 billion) and a record loss for the year of Swfr19.7 billion.

UBS shares crashed to an all-time low this week amid deepening concerns over the tax fraud probe in the US.

The authorities in Washington have been pursuing the bank over advice and help it gave to US citizens to evade taxes.

The bank last week agreed to pay a fine of $780 million (£548 million) and give up the identities of about 300 of its US clients — ending Switzerland's cherished reputation for banking secrecy.

The bank has cut a total of 9000 jobs since the credit crunch hit, including about 600 at its City offices in Broadgate where it has 7000 employees.

It is axing a further 2000 jobs worldwide as it reels from its record losses, with several hundred expected to come in London. Bonuses have been reduced by more than 80%.

UBS, the world's largest manager of money for the wealthy, is planning to spin off $39.1 billion in toxic assets to the Swiss central bank as part of the government rescue package it hopes will return it profit.