The CEO of the world’s largest ad agency network, Sir Martin Sorrell, has stepped down from his post, effective immediately.

Sorrell’s resignation from WPP comes after the company announced an investigation into “alleged personal misconduct” against him.

“The previously announced investigation into an allegation of misconduct against Sir Martin has concluded,” a statement by WPP said.

“The allegation did not involve amounts that are material.”

Sorrell had “unreservedly” rejected the allegation that he misused company assets.

WPP chairman Roberto Quarta has become executive chairman until the appointment of a new CEO.

Mark Read, CEO of Wunderman and WPP Digital; and Andrew Scott, WPP’s corporate development director and chief operating officer for Europe; have been appointed as joint chief operating officers of WPP. Sorrell will be available to assist with the transition.

Adland’s most well-known figure said in a statement he was sad to leave WPP after 33 years.

“It has been a passion, focus and source of energy for so long. However, I believe it is in the best interests of the business if I step down now,” Sorrell said.

“I leave the company in very good hands, as the board knows. Mark and Andrew and the management team at all levels have the knowledge and abilities to take WPP to even greater heights and capitalise on the geographic and functional opportunities.

“I will particularly miss the daily interactions with everyone across the world and want to thank them and their families for all they have done, and will do, for WPP.”

Quarta said Sorrell has been the driving force behind the expansion of WPP to create the global leader in marketing services.

“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.

“On behalf of the board, I would like to recognise these achievements and thank Sir Martin for his commitment to the business over more than three decades.”

In accordance with his at-will employment agreement, WPP said Sorrell will be treated as having retired on leaving the company, as detailed in the directors’ compensation policy.

“His share awards will be pro-rated in line with the plan rules and will vest over the next five years, to the extent group performance targets are achieved,” WPP said.