Shares in WPP, the advertising agency giant, have fallen after an announcement that the board is investigating an allegation of personal misconduct against its chief executive, Martin Sorrell.

In early-afternoon Wednesday trading in London, WPP's share price was down 2.2 percent at 1,094 pence.

The Wall Street Journal reported the allegations involve possible misuse of assets and improper behavior.

No details were offered but WPP says "the allegations do not involve amounts which are material to WPP."

Sorrell, who has run WPP for more than three decades, noted the claim of financial impropriety and said he rejected "the allegation unreservedly," adding that he won't play any role in the management of the investigation.

Ian Whittaker, analyst at Liberum, says the allegations could "intensify" speculation about Sorrel's future as CEO.