Sorrell, who worked at Saatchi & Saatchi before founding WPP, said the company has been "a passion, focus and source of energy for so long".
The company's statement is also expected to say that Sir Martin will remain available to assist with the transition to new leadership, and add that the investigation into the allegation against him has now concluded, they added. Meanwhile, CEO of Wunderman and WPP Digital, Mark Read, as well as WPP corporate development director and COO, Europe, Andrew Scott, have been appointed as joint COOs of WPP. "And our highly talented people have always won through, always", said Sorrell. I leave the Company in very good hands, as the Board knows.
Nobody, either direct competitors or newly-minted ones can beat the WPP team, as long as you work closely together, whether by client and/or country or digitally.
Meanwhile, Roberto Quarta, chairman of WPP, will become executive chairman until the appointment of Sorrell's full-time successor is made. 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.
Quarta said: "Sir Martin has been the driving force behind the expansion of WPP to create the global leader in marketing services".
Sorrell has resigned under a cloud and his PR Companies are working overtime to protect the reputation of the world's largest advertising and communications group.
In his time the group expanded to own top creative agencies including J. Walter Thompson and Young & Rubicam, as well as media planners and buyers, market-research firms and public relations groups such as Finsbury.
WPP PR Companies operating in Australia include Ogilvy PR, Burson Marsteller, Hill & Knowlton, PPR, Pulse and several conflict agencies.
Along the way as his legend grew - so did the amounts he took as payments for his hard labour which caused heartburn to investors and shareholders.
Sorrell, one of Britain's best-known businessmen, denied any wrongdoing after the allegations surfaced earlier this month, but said he understood the company had to investigate.
A spokesperson for WPP provided a statement, attributed to Sorrell.
Said he: "I shall miss all of you greatly".
"If WPP does well, I do well", he told the Press Association in April 2016.
Analysts believe Sorrell's departure combined with a share price slump of nearly a third over the past year means investors and the WPP board will have to consider that there is potentially more value in breaking up WPP.