Stephen Volk, chairman of Credit Suisse First Boston and close ally to its former chief executive John Mack, has quit to take on a senior role at Citigroup.

Volk is the most senior CSFB manager to leave the bank since the departure of Mack on July 12. He is joining Citigroup as vice-chairman and client adviser with a remit to help grow the bank's corporate and investment banking division.