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.
The investigation into Sorrell has now concluded, WPP said, reiterating that the amounts weren't material.
In the past few years, his role has come under pressure in context to his pay and other issues, including the drop in WPP revenue last year.
Sorrell said in a statement: "Obviously I am sad to leave WPP after 33 years".
WPP released a statement stating that the change would become effective immediately and that the advertising behemoth's chairman Roberto Quarta would become executive chairman until the company could appoint a new chief executive.
Mr Sorrell is among the most influential advertising professionals globally and also the longest-serving FTSE-100 chief executive, who has run WPP since setting up the business in 1985.
One person familiar with the process said the investigation was unlikely to be resolved before the end of next week.
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"His departure will leave the company he built virtually from scratch facing profound questions about its future direction", he said.
Quarta called Sorrell "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.
Born in London, Sorrell studied economics at the University of Cambridge and then gained a masters from Harvard University.
Born to Jewish parents who were from Kiev, Sorrell began his career in the renowned and trendy ad agency Saatchi & Saatchi (which fashioned winning electoral campaigns for Britain's conservative prime minister Margaret Thatcher) in 1975. Queen Elizabeth II knighted him in 2000 onto his bum, the maximum honor one of the raft of awards he's received through recent several years.
The media empire has kicked off a hunt for a new boss after the 73-year-old advertising king abruptly quit after growing "fed up and p***d off" during an investigation into alleged misconduct.
However, he always fiercely defended his income, saying it was related to how well the company he started from nothing was doing. He earned about 200 million pounds ($284 million) over the last five years, largely due to a lucrative performance-related bonus package.