Former WPP CEO Martin Sorrell has responded to allegations that the ad holding company investigated his use of company money for a prostitute before he stepped down.

The subject matter of that investigation was discussed in a report this weekend in the Wall Street Journal.

"WPP has been advised that it cannot disclose details of the allegations against Sir Martin Sorrell because it is prohibited by data protection law from giving such details," a WPP spokesman said in a statement. "Sir Martin chose to resign at the conclusion of the investigation by independent legal counsel."

A spokesman for Sorrell said the former WPP executive is prohibited from discussing the circumstances around his resignation.

"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," the spokesman said. "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."

In early April, WPP confirmed its board was investigating Sorrell in response to undisclosed alleged "personal misconduct." The executive said he denied the allegation "unreservedly."

But just two weeks later, Sorrell and WPP announced that he would be stepping down in the interest of the company, clients, shareholders and all other stakeholders. At the time, WPP said its investigation had concluded and did not "involve amounts that are material."

"In accordance with his at-will employment agreement, Sir Martin will be treated as having retired on leaving WPP, as detailed in the Directors' Compensation Policy," WPP said at the time. "His share awards will be pro-rated in line with the plan rules and will vest over the next five years, to the extent Group performance targets are achieved."

Sorrell's departure set off a swirl of questions of what would have prompted the investigation, but WPP has remained quiet about the details.

At an investor meeting in London in late April, asked about the circumstances surrounding Sorrell's departure, WPP executive chairman Roberto Quarta said Sorrell had resigned "before the board had taken into consideration the outcome of the investigation and determined whether or not it was appropriate to take action."

"We have no requirements to disclose or necessity to disclose," he said. "I think that we also make it very clear … that the financial impact on this personal misconduct allegation was not material to the WPP earnings. That's pretty much the case."

He added that since it is considered to be personal conduct, it was a "matter of privacy, and therefore, it's a matter for Martin, and hence the reason why we did not disclose."

WPP's annual general meeting is scheduled for Wednesday in London.