Shares in WPP fell in early trading after the firm put out a statement this morning saying it had appointed independent counsel regarding an allegation of personal misconduct against its chief executive.
Sir Martin Sorrell has said he rejects the allegation "unreservedly".
Shares in the firm dipped more than three per cent in early trading.
In a statement put out in the early hours of this morning, WPP's chief executive said:
Reports in the media have stated that WPP is investigating an allegation of financial impropriety by me, specifically as to the use of company funds.
This allegation is being investigated by a law firm.
I reject the allegation unreservedly but recognise that the company has to investigate it. I understand that this process will be completed shortly.
Obviously, I shall play no part in the management of the investigation under way.
The boss of the advertising behemoth said that as a significant shareholder, his commitment to the firm which he founded over 30 years ago "remains absolute - to our people, our clients, our shareholders and all of our many stakeholders".
"I do not intend to make any further statement at this time," he added.
The allegations were first reported by the Wall Street Journal, and said the board was also examining whether Sorrell misused company assets.
Sorrell has built a longstanding reputation as a formidable business leader, building WPP into a company that is now the world's biggest marketing communications firm. He has been at the helm of WPP for more than 30 years.
Earlier this month, WPP reported its worst year in growth terms since 2009, with annual revenues dipping by 0.9 per cent to £15.2bn. The firm said it would accelerate a strategic overhaul of its business and simplify its structure.