Advertising firm WPP (LON:WPP) saw shares sink 2.5 percent at market open on Wednesday, after the group announced a misconduct investigation against its chief executive.

The group announced that Sir Martin Sorrell was under investigation in a statement on Tuesday, appointing an independent council to the process. The group added, however, that “the allegations do not involve amounts which are material to WPP”.

Sorrell said on Wednesday that he rejected “unreservedly” the reports of financial impropriety.

He added that he recognised ‘that the company has to investigate it”, and that the “process will be completed shortly.”

Martin Sorrell is the biggest paid chief executive in UK history, after forming the advertising giant WPP in 1985. He has overseen the growth of the company, which now has 3,000 offices in 112 countries.

Shares in WPP are currently trading down 2.58 percent at 1,088.68. The company’s share price has taken a hit of late, after it warned that 2017 was “not a pretty year” for the firm.