WPP also revealed Saturday however that the probe had concluded, adding in a company statement that "the allegation did not involve amounts that are material".
Sorrell spent 33 years at the helm of the business but is leaving after an internal investigation into allegations of misconduct and misuse of assets.
Sorrell in the letter he wrote to his employees said that the current disruption has caused the business to come under too much unnecessary pressure.
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.
"To me it's not actually about whether he did anything wrong but it's the fact that three years ago the board would not even have gone down this path", the former CEO said. There is no way on earth that WPP would want to look at that short list of candidates and in essence pick someone to replace Sir Martin who was rejected by Publicis! When Sorrell bought a 30 percent stake in Wire and Plastic Products, a shopping basket manufacturer, in 1985, there was little to suggest that it would go on to become the world's leading ad agency.
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Sorrell was an outspoken and recognizable figure across the advertising world and recently gained much attention for the huge cut in pay he took as WPP faced its worst prospects for growth in almost a decade. Respected and sometimes feared by all sides.
"He is so brutally competitive", the former CEO said.
WPP has started the search for a new chief executive.
Energetic and competitive, Sorrell is widely regarded as the titan of his industry known for his combative style, especially when pointing out the failings of his competitors to journalists.
"He has an astounding grasp of detail if you consider the scale of the enterprise", said another executive who knew Sorrell professionally for many decades.
Sorrell was very well compensated as chief executive, earning circa £210 million (nearly $300 million) over 5 years.
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In March, the group reported its worst full year performance since the financial crisis and forecast no growth for 2018, reflecting a reduction in spending from consumer goods giants.
Whoever takes over will have to decide whether the group of 200,000 people should remain in its current form.
During his tenure, WPP acquired top advertising agencies such as J. Walter Thompson and Young & Rubicam, as well as companies specializing in media planning, market research and PR, such as Finsbury. "Sir Martin still owns circa 2% of the shares". Mark Read, CEO of WPP Digital, and Andrew Scott, COO of WPP's European division, have been appointed joint chief operating officers of WPP on an interim basis.
Sorrell told staff that WPP had come through hard times before and would do so again, saying he would be available to anyone who wanted advice.
Sorrell will be available to assist with the transition, WPP said. "Now back to the future".
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