The departure of Sorrell, who resigned on Saturday before learning the findings of an internal investigation into alleged personal misconduct, is also being viewed as a potential catalyst for a break-up of WPP.
Roberto Quarta, chairman of WPP, moves into the executive chairman slot until a replacement for Sorrell is announced.
"Obviously I am sad to leave WPP after 33 years". It has been a passion, focus and source of energy for so long.
In a statement, he said it was "in the best interests of the business if I step down now". "For the past 33 years, I have spent every single day thinking about the future of WPP", wrote Sorrell.
WPP CEO Martin Sorrell's resignation preempts what could have been an inglorious departure for someone who was the public face of the advertising industry writ large.
"The succession arrangements look like a sticking plaster and, until strong and credible long-term leadership arrangements emerge, there will be concerns about the sustainability of the WPP Group, as we know it today, and its strategic direction", said Guy Jubb, former global head of governance and stewardship at Standard Life Investments, an investor in WPP.More news: Chong Wei beats world No 1 in Commonwealth singles
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Sorrell, a former top advertising executive for Saatchi & Saatchi, developed the group after buying a small British plastics company, Wire & Plastic Products.
However, Sorrell will assist with the transition as the company seeks a new leader. However, I believe it is in the best interests of the business if I step down now.
"We have weathered hard storms in the past". 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. "And our highly talented people have always won through, always", said Sorrell. I leave the Company in very good hands, as the Board knows.
The longest-serving CEO on the FTSE 100 blue chip index, Sorrell built WPP into one of Britain's biggest companies by three decades of relentless dealmaking.
"In the coming period, I will be available to the board and any of you, should you want help with anything, anywhere", he said.
He added: As a significant share-owner, my commitment to the company, which I founded over 30 years ago, remains absolute - to our people, our clients, our shareholders and all of our many stakeholders. "Good fortune and Godspeed to all of you".