Top 10 shareholder Aberdeen Standard Investments, the only UK institution to hold a significant stake in WPP, is in talks with the business on improving its practice, City A.M. understands.
Meanwhile, shareholder advisory group Glass Lewis has told shareholders to vote against the companys remuneration report and the re-election of chairman Roberto Quarta at the companys 13 June AGM.
Glass Lewis said it has "severe reservations" about certain issues at the company, including the failure to publish a report detailing the alleged personal misconduct which prompted Sorrell's resignation.
Sorrell was classed as a "good leaver" after he suddenly left the company in April, meaning he will still get up to £20m in future share payouts.
Quarta came under fire as Glass Lewis was not convinced about the "transparency and efficacy of the succession process" to replace the CEO.
WPP has maintained that ousting the chairman would not be in the best interests of investors, and said that Sorrell had been "treated in accordance with his contract and also in accordance with the approved compensation policy and the stock plan rules".