WPP said the probe had concluded, adding that "the allegation did not involve amounts that are material".
Sir Martin's future will also be watched with interest - his parting message to staff was "back to the future" and the FT reported that he had never signed a non-compete agreement, leaving him free to being a new advertising venture.
New-York based Omnicom Group Inc., another WPP competitor, recently consolidated the chief executive officer and chairman role in John Wren, who has led the company for two decades. "It has been a passion, focus and source of energy for so long".
"That is why I have decided that in your interest, in the interest of our clients, in the interest of all share owners, both big and small, and in the interest of all our other stakeholders, it is best for me to step aside".
Some analysts believe WPP, which comprises some 400 separate businesses including Ogilvy & Mather, Kantar Group, Hogarth Worldwide and Young & Rubicam, could be broken up.
Chemical inspectors to begin work in Syria's Douma
United States envoy Nikki Haley said the strikes were "justified, legitimate and proportionate". But they themselves seem to have skirted domestic and worldwide laws to do so.
"Yes he was relentless and richly rewarded - but let's not forget Sir Martin Sorrell built a global advertising empire from nothing", said Lionel Barber, editor of the Financial Times newspaper.
Sorrell is a titan of British business who was named the world's second-best performing CEO in 2017 by the Harvard Business Review.
Present in 112 countries, WPP serves clients including Ford, Unilever, P&G and a string of major corporations around the world.
He previously worked at Saatchi & Saatchi, and was knighted in the Queen's New Year honours list in 2000. Its shares are down around 30 percent this year.
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.
"As a founder, I can say that WPP is not just a matter of life or death, it was, is and will be more important than that".
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