Sir Martin would of course assist with the transition.
Sorrell is one of the UK's best-paid business leaders, earning more than £200m from pay and lucrative - and highly controversial - reward schemes in the past five years alone.
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.
WPP's directors have been in frequent contact by telephone since the allegations emerged and people close to the company played down Tuesday's face-to-face board meeting.
Sorrell, via a statement from his crisis PR rep Project Associates, said he's sad to leave WPP.
Sky News has learnt that WPP, which owns a string of worldwide advertising and media buying networks supplying numerous world's biggest companies, is preparing to announce Sir Martin's departure as soon as Saturday night. It has been a passion, focus and source of energy for so long.
Mark Read, chief executive of Wunderman and WPP Digital, and Andrew Scott, WPP corporate development director and chief operating officer, WPP Europe, have been appointed as joint chief operating officers of WPP. It made no further comment but repeated a previous statement that the allegation did not involve amounts that were material to the company. I leave the Company in very good hands, as the Board knows.More news: Extra glass of wine, beer takes 30 minutes off your life
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The ad giant has been under pressure since it was forced to admit this month that it had launched...
Chairman Roberto Quarta takes over as executive chairman. Sorrell, who will help with the handover to a new chief executive, denies wrongdoing.
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.
Several people who have attended WPP board meetings in the past identified Sol Trujillo, a former US telecoms boss, as a non-executive director who sometimes asked the most challenging questions.
The probe served to intensify scrutiny on Sorrell and WPP's succession planning, long criticized by investors as being insufficient, prompting speculation that a CEO long seen as unstoppable might be forced to step aside.
WPP enjoyed a strong period of growth from the end of the global financial recession in 2008-09 until the end of 2016.
"As a founder, I can say that WPP is not just a matter of life and death, it was, is and will be more important than that".