WPP’s long-serving chief executive Sir Martin Sorrell has resigned from his role as the head of the company amid an investigation into an alleged misuse of assets and improper conduct.

The decision to step down, which has happened with immediate effect, comes after WPP hired an independent counsel to launch a probe into accusations levelled at Sorrell – though they are not financially material to WPP.

Sorrell’s abrupt departure has left Roberto Quarta, chairman of WPP, to step into the role in the interim while the company searches for a new CEO. Mark Read, CEO of Wunderman and WPP Digital, and Andrew Scott, WPP corporate development director and Europe COO, have been made joint COOs of the company.

“Obviously I am 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,” said Sorrell.

“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.”

Sorrell had been in charge of the advertising giant since 1986, after he made an investment in wire shipping basket company Wire & Plastic Products a year earlier. He then transformed that business into the advertising behemoth we know today with 3,000 offices in 112 countries.

As a result of his decision to step down, Sorrell will be treated as being retired as per his employment agreement. He will remain available to WPP to aid it in the transition process.