The UBS takeover of Credit Suisse comes with a huge amount of risk, UBS chairman Colm Kelleher admitted Wednesday after announcing Sergio Ermotti would return as CEO to handle the merger.

Ermotti was the chief executive of Switzerland's biggest bank from 2011 to 2020 and Kelleher said the board felt he would be a "better pilot" than current CEO Ralph Hamers to oversee the transition.

"There's a huge amount of risk in integrating these businesses," Kelleher told a press conference in Zurich.

UBS and Credit Suisse, the second-biggest bank in Switzerland, were both among the select banks around the world considered to be global systemically important financial institutions (G-SIFIs).

Kelleher said it was "the biggest single financial transaction" since the 2008 global financial crisis.

"I would argue it's bigger than any deal that was done in 2008, because it's the first time two G-SIFIs have merged. That brings with it significant execution risk," he said.

"I cannot re-emphasise how big this deal is in terms of financial history and financial engineering."

The shotgun marriage of UBS and Credit Suisse was hastily arranged by the Swiss government on March 19 to prevent a global financial meltdown following fears of contagion from the collapse of banks in the United States.

Kelleher said there were clearly parts of Credit Suisse that had a "bad culture", primarily in its investment banking, and he did not want to import that into UBS -- though he said its retail banking and wealth management were "probably really quite clean".

"Investors, our shareholders, by and large, see significant upsides in this transaction," he explained.

"But they are very concerned about execution risk and we have a lot of execution risk here, so this is not in any way an easy deal to do," the chairman said on reasons behind the change of CEOs.

Ermotti will take up his post on April 5, with Hamers continuing on for a transition period.

"It was the opinion of the board that for this massive integration exercise, Sergio would be the better pilot for this next voyage of UBS," said Kelleher.

Ermotti said he felt the "call of duty" to return.

"The debate is not too big to fail, rather it's too small to survive, and we want to be a winner out of this," he said.