Martin Sorrell, the CEO of the advertising giant WPP, is under investigation by an independent law firm because of "an allegation of personal misconduct," the company says.

The company does not describe the nature of the misconduct, but says the allegations "do not involve amounts which are material to WPP." The company had revenues of $21.4 billion in 2017.

The Wall Street Journal, citing unnamed sources, reports that the allegations involve "misused company assets" as well as "personal behavior."

Sorrell, 73, says he unreservedly rejects the allegation of financial impropriety. In a statement, he says he understands the company must investigate and that he believes the process will be completed shortly.

"The investigation is ongoing," WPP says.

Sorrell is a giant in the global advertising industry, and one of Britain's most prominent businessmen. He's famously well-compensated, earning £210 million (nearly $300 million) over the course of 5 years in a controversial pay package that some shareholders resisted.

Within the last year, he's come under pressure because of his company's performance — WPP has seen poorer-than-expected growth and slumping stocks.

Sorrell, formerly the finance director at Saatchi & Saatchi, created WPP in the mid-'80s. He invested in a manufacturing company called Wire and Plastic Products Plc, took over as chief executive and converted the company into a marketing firm. Through acquisition after acquisition, the renamed WPP Group grew from a small operation into a behemoth.

Today, it is the world's largest advertising company, and Sorrell has been called the world's "most important advertising executive."

WPP owns hundreds of ad and PR agencies around the world, which shape the image of "the Who's Who of the Fortune Global 500," as NPR's Alina Selyukh has pu t it. WPP clients include Ford, Proctor & Gamble, Nestle and Unilever.

But, as Alina reported, WPP has struggled to keep up with massive changes to the advertising industry in the Internet era.

Sorrell isn't just the face of WPP. The man and the company are tightly tied together, and have been from the start. Think Mark Zuckerberg and Facebook, Steve Jobs and Apple, Warren Buffett and Berkshire Hathaway.

In 2017, Sarah Wilson, the head of a shareholder advisory service, told The Guardian that Sorrell "identifies very closely with WPP as his personal creation." Then she warned: "The cult of the superstar chief executive does not create a healthy corporate culture. They're increasingly the only person we associate with the organisation, its successes or failures."

Sorrell has never identified a protege or begun grooming a successor, and has never indicated plans to retire. Bloomberg writes that "a WPP without Sorrell at the helm still seems unimaginable."

But unimaginable does not mean impossible. Unlike Silicon Valley titans, Sorrell "does not own a controlling stake" in his company, the Financial Times notes.

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