Stan O'Neal, chairman and chief executive of Merrill Lynch, has forced the departure of Arshad Zakaria. head of global markets and investment banking, in a bid to tighten his grip on power at the helm of the firm.

Zakaria is to leave the bank at the end of the year. His departure follows a boardroom row over attempts to have Zakaria named president of Merrill Lynch and O'Neal's official successor.