In a brief statement, the marketing services group said that the investigation was ongoing and that the allegations did not involve amounts "which are material to WPP".

The investigation appears to involve the improper use of company funds.

Sorrell said in his own statement: "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."

Sorrell is one of the best known business leaders, having founded WPP in 1985 and built it into the largest operator in this industry. He is also one of the highest paid chief executives in the UK.

WPP recently reported a more difficult trading year, with its lowest growth since the 2008/9 recession.