Sorrell has not denied that the investigation covered the subject, but has issued a public statement through a spokesman denying specific allegations that appeared in The Wall Street Journal this weekend.
The spokesman declined to comment on a separate story carried today by The Times, which also contains sensitive allegations.
A friend of Sorrell told Campaign that Sir Martin was unaware of what the investigation was about because he was not shown it and its findings have not been published.
WPP’s stance is that it cannot divulge details of the allegations against Sorrell that the investigation covered because it is prohibited by data protection laws from giving such details.
The friend of Sorrell said the investigation related to an alleged single payment.
The friend said: "He denies the allegation that he saw a prostitute. He understands that the allegation was made by a disgruntled employee. If you run a business with 200,000 employees for 30 years, some will fall out with you."
Sorrell’s spokesman said: "Sir Martin signed a non-disclosure agreement when he stepped down which precludes him from discussing any of the circumstances surrounding his departure. He has rigidly adhered to this obligation and will continue to do so. As regards the allegations which have appeared in The Wall Street Journal, Sir Martin strenuously denies them. He will be making no further comment at this time."
Sorrell resigned from WPP in April.