In a statement issued through a communications consultancy working for Sorrell in a "personal capacity", Sorrell describes the allegation as "financial impropriety by me, specifically as to the use of company funds".
Many executives recount stories of the CEO taking contract losses personally, including one who told Reuters how Sorrell had shared an hour's vehicle journey in complete silence after his rival mentioned an account he had recently won from WPP.
According to research from the High Pay Centre think-tank, Sorrell was Britain's best-paid boss in 2015, with a package of more than £70 million ($100 million, 80 million euros) that year.
"Mark and Andrew and the management team at all levels have the knowledge and abilities to take WPP to even greater heights and capitalize on the geographic and functional opportunities", said Sorrell. "However, I believe it is in the best interests of the business ...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....the current disruption is putting too much unnecessary pressure on the business".
Sorrell will be treated as having retired, based on the directors' compensation policy, the company said.
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WPP plans to announce that Mark Read, the chief executive of Wunderman, an agency network it owns, and Andrew Scott, chief operating officer of the group's European business, will take over as interim joint chief operating officers. "It has been a passion, focus and source of energy for so long", stated Sorrell in an emotional note to WPP staff.
Sorrell has been for personal misconduct since April 3.
Sir Martin Sorrell is stepping down as CEO of the marketing services group WPP, Sky News can reveal. I leave the Company in very good hands, as the Board knows.
The departure of one of Britain's best-known businessmen leaves the advertising giant needing fresh leadership at a testing time for the marketing industry, with social media companies offering brands a direct connection with vast audiences.
"Most of my wealth, if not all of it, is and has been for the last 31 years tied up in the success of WPP".
He added: "We have weathered hard storms in the past. And our highly talented people have always won through, always", said Sorrell. I shall miss all of you greatly. WPP says that Sorrell "will be available to assist with the transition". "I think most analysts and investors when adding up the sum of the parts of WPP would answer that it is to break it up", said Alex DeGroote, an analyst with Cenkos Securities. "Good fortune and Godspeed to all of you...now Back to the Future".