Martin Sorrell, who built WPP into the world's biggest advertising agency through 33 years of dealmaking, quit Saturday after an allegation of personal misconduct. The departure of the CEO who built a two-man outfit into one of Britain's biggest companies with 200,000 staff in 112 countries leaves WPP without a boss at a pivotal time for the industry and when the group is under great strain. WPP stunned the market last week when it said it had appointed lawyers to investigate alleged misconduct by Sorrell. He denied the allegations but in a letter to WPP staff published late Saturday he said the "current disruption" was "putting too much unnecessary pressure on the business." He said he had decided that "in your interest, in the interest of our clients, in the interest of all shareowners, both big and small, and in the interest of all our other stakeholders, it is best for me to step aside."