The head of the world's largest marketing group, WPP, has announced his resignation following an investigation into his alleged misuse of funds. Martin Sorrell said he was resigning with immediate effect as chief executive of WPP, which he founded in the 1985, "in the best interests of the business." "Obviously I am sad to leave WPP after 33 years. It has been a passion, focus and source of energy for so long," Sorrell, 73, said in a statement issued by WPP, dated Saturday.

"However, I believe it is in the best interests of the business if I step down now," he said. "I leave the company in very good hands, as the board knows."

WPP said its "investigation into an allegation of misconduct" against Sorrell had concluded.

"The allegation did not involve amounts that are material," it said.

Sorrell had denied allegations of misuse of company funds and improper personal behavior, according to British media reports. WPP said group chairman Roberto Quarta would replace Sorrell as chief executive until a long-term replacement is found.

"Sir Martin has been the driving force behind the expansion of WPP to create the global leader in marketing services," Quarta said. "During this time, the company has been successful because it has valued and nurtured outstanding talent at every level - within and well beyond our leadership teams," he said. Sorrell, a former top advertising executive for Saatchi & Saatchi, developed the group after buying a small British plastics company, Wire & Plastic Products.