The toppling of even one Wall Street titan would normally be enough to send shockwaves through the world’s financial boardrooms. Yet this year no less than three chief executives of bulge-bracket banks must be peering nervously over their shoulders, wondering when, not if, the axe will fall.

Yesterday the heat was not only on Stanley O’Neal, chief executive of the mighty Merrill Lynch, but also his counterparts at Citigroup and Bear Stearns. All have faced a welter of criticism from investors and analysts as their banks have reeled from the effects of the credit crunch. So far, the three are hanging on, but few Wall Street-watchers would be surprised if it wasn’t all change at the top by the end of the year.

Speculation was rife yesterday that Mr O’Neal would lose his job after he was reported to have entered merger talks with Wachovia without prior approval from Merrill Lynch’s board. Mr O’Neal’s apparent blunder came on top of an announcement this week that the bank had been forced into a $5.5 billion writedown in the third quarter as a result of the credit crisis. It was the bank’s first quarterly loss in six years, a setback that left investors more than a little disappointed.

Mr O’Neal had tried to avert shareholder fury this month by changing the leadership of the bank’s fixed-income business. Osman Semerci, the London-based global head of the bank’s fixed-income, commodities and currencies (FICC) division, left the bank, along with Dale Lattanzio, head of the division’s US operations. But it may not have been enough.

Yesterday it was reported that Mr O’Neal’s replacements were being lined up. Front-runners are thought to include Laurence Fink, the chief executive of BlackRock, Merrill Lynch’s fund management business, and John Thain, the chief executive of the New York Stock Exchange.

Laying off the underlings in charge of loss-making divisions was a ploy also used, with limited success, by Charles Prince. Two weeks ago the chief executive of Citigroup announced a radical shake-up of the bank’s top management, in which he sacked Randy Barker, the co-head of Citigroups fixed-income business, and moved Mr Barker’s co-head, Geoff Coley, to another job. Tom Maheras, who ran Citigroup’s capital markets operations, including the division run by Mr Barker, also left, apparently out of loyalty to Mr Barker.

The changes did not, however, halt the calls for Mr Prince’s head, despite assurances from Bob Rubin, the head of Citigroup’s executive committee, that Mr Prince would remain in his job for several years to come. Analysts at Bear Stearns said that if the reshuffle did not improve the bank’s performance, Citigroup’s board would have “to conclude that a more significant change, at CEO level, is required”. There have also been questions abut the tenure of Jimmy Cayne, the chief executive of Bear Stearns. Investor discontent with the management team has been widely reported and that feeling was unlikely to have been helped by Bear Stearns’s decision to close two of its hedge funds this year after the pair lost $1.5 billion between them. Last month the bank reported a $700 million writedown on the value of its mortgage holdings and leveraged loan commitments in the third quarter.

Mr Cayne tried to buy himself some time with the announcement of a joint venture with Citic Securities, China’s biggest listed brokerage, but rumours that the bank needed a capital injection or could be subject to a sale or break-up have continued. Bear Stearns’s chief executive has recently been away from the office because of ill-health and, with expectations in some quarters of a fourth-quarter writedown still to come, there has been speculation, including from inside the bank itself, that it was time for the 73-year-old boss to step aside.

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