The advertising guru earlier this year left WPP – the company he founded more than 30 years ago – following allegations of personal misconduct.
WPP carried out an inquiry into allegations Sir Martin misused company funds, but the details of the investigation were never disclosed.
It was alleged by the Wall Street Journal over the weekend that Sir Martin had used £300 in company cash to pay for a sex worker.
However, Sir Martin’s spokesman “strenuously” denied the claims.
A spokesman for the tycoon 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.”
WPP has said it cannot disclose details of the allegations because it is prohibited by data protection law from doing so.
The allegations come ahead of what is expected to be a fiery WPP annual meeting on Wednesday where Sir Martin will again be the focus.
Investors will vote on key appointments and the advertising giant’s pay report.
But two influential shareholder advisory groups, Glass Lewis and PIRC, are recommending investors reject the group’s remuneration report, which includes a payout to Sir Martin.
Glass Lewis has said that without more information from WPP, shareholders were unable to determine whether Sir Martin was a “good leaver”, adding that it has “severe reservations” about the firm’s pay plans, given the lack of transparency.
Sir Martin is in line to receive £14 million from WPP, having received £48.1m the year before.
He will also hoover up nearly £20m in payouts from WPP over the next five years as part of an exit deal with the firm.
Sir Martin earlier this month confirmed his return to the London stock market, with the former WPP boss to head up the newly formed S4 Capital.