Hell hath no fury like a top executive scorned.

Luqman Arnold, who was ousted as president of UBS in 2001, is putting new pressure on the Swiss banking giant, which has repeatedly stumbled during the credit crunch.

Arnold's London-based investment firm, Olivant, has built up a 0.7 percent stake in the bank and is urging UBS to split off its investment banking unit and remove its recently appointed chairman.

Arnold sent a letter on Thursday to Sergio Marchionne, UBS' vice chairman as well as the chief executive of Fiat, urging the board to consider a shake-up.

Two days earlier, UBS announced a $19 billion write-down on U.S. real estate and "related structured credit positions." With that write-down, the bank has marked down some $37 billion worth of assets since just October.

As a result, UBS' longtime chairman, Marcel Ospel, is being forced out, to be replaced by Peter Kurer, once the bank's general counsel.

But Arnold says that Kurer's appointment "perpetuates UBS' ineffective corporate governance and insular culture."

Arnold is also questioning the structure of UBS, echoing criticism of another huge bank, Citigroup, from investors and former executives who have argued that the "financial services supermarket" model does not work.

"We are not convinced that the 'one bank' integrated business model that has served UBS well in the past will survive the damage inflicted by the proprietary trading losses and write-downs," Arnold wrote.

In an interview with CNBC today, Arnold clarified that he was not necessarily calling for the bank to be broken up but that it could improve through a restructuring.

The move by Olivant will step up the pressure already on UBS from disappointed investors.

The Financial Times' Lex column says that Arnold's experience at UBS and his reputation "raises Olivant's campaign above the usual humdrum activism. Not only will the board have to take Mr. Arnold seriously, exasperated shareholders will have somebody to rally around."

Patrick Hosking of the Times of London agrees that Arnold will have to be taken seriously by UBS.

Noting that the full details of Arnold's ouster in 2001 have not come out, he says that "Luqman Arnold will emphatically not want his assault on UBS to be seen as revenge. But there is something more than just coincidental about his appearance on the UBS shareholder register just as his nemesis, the outgoing UBS chairman, Marcel Ospel, clears his desk."

UBS acknowledged today that it had received the letter and would respond at a later time. Shares of UBS rose more than 3 percent in trading in Switzerland.