The boss of the world’s largest advertising network is under investigation for an allegation of “personal misconduct”.

According to a statement issued by WPP, the company has appointed “independent counsel to conduct an investigation in response to an allegation of personal misconduct” against CEO Sir Martin Sorrell.

“The investigation is ongoing. The allegations do not involve amounts which are material to WPP,” the statement said.

The Wall Street Journal first reported the allegations against Sorrell. It also reported that WPP’s board was also examining whether he misused company assets.

Sorell, who is 73, has been serving as CEO of WPP for more than three decades.