WPP is investigating an allegation of personal misconduct against its chief executive Sir Martin Sorrell.
The world's largest advertising agency said its board had appointed independent counsel to conduct the investigation.
"The allegations do not involve amounts which are material to WPP," the firm said in a statement on Tuesday.
Mr Sorrell responded on Wednesday, saying he rejected any allegation of financial impropriety "unreservedly".
"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," Sir Martin said in a statement.
"This allegation is being investigated by a law firm. I reject the allegation unreservedly but recognise that the company has to investigate it," he said.
"I understand that this process will be completed shortly."
Sir Martin added: "As a significant share owner, my commitment to the company, which I founded over thirty years ago, remains absolute - to our people, our clients, our shareholders and all of our many stakeholders. "
Who is Sir Martin Sorrell?
- Formed WPP in 1985 after taking control of a shell company, Wire & Plastic Products, and established it as a marketing services group in 1986
- Oversaw growth of company which now has 3,000 offices in 112 countries
- One of UK's top-paid executives. His pay package in 2015-16 was £70m - then the biggest in UK corporate history - which more than a third of investors refused to back
- A lack of succession planning for after the 73-year-old's retirement has caused some anxiety among investors
Share price falls
The allegations about Sir Martin were first reported on Tuesday by the Wall Street Journal.
The Journal also reported that WPP's board was examining whether Sir Martin misused company assets, citing unnamed sources.
That issue was not addressed in the company's statement, which only confirmed an investigation into the personal misconduct allegation.
One of the UK's best-known business leaders, Sir Martin has built WPP into a firm that dominates the advertising industry and has about 134,000 staff in more than 100 countries.
He has been chief executive of the company for more than three decades.
Last month WPP warned that 2017 was "not a pretty year" for the firm due to flat growth, despite profits of more than £2bn.
Its share price has fallen sharply over the past 12 months from £17.31 to close at £11.17 in London on Tuesday.