Quarta took on the role of executive chairman at the company following the departure of longstanding chief executive Sir Martin Sorrell last month.

Glass Lewis said in a report to investors that it had “severe reservations” over issues including the failure to publish the outcome of the investigation into personal misconduct that led to Sorrell’s resignation, the Guardian reported.

The advisory body also advised investors to vote against the company’s pay report because of the lack of transparency around Sorrell’s departure, and criticised the lack of succession planning. 

“We harbour concerns as to the transparency and efficacy of the succession process,” said Glass Lewis. “Despite previous assurances we believe the nomination committee has failed to adequately prepare for the replacement of Sir Martin. Our concerns are heightened by the opaque nature of the investigation into Sir Martin [and] his ‘good leaver’ status. Absent further information regarding Sir Martin’s retirement, we believe shareholders are unable to determine the extent to which he should be treated as a ‘good leaver’.”

A WPP spokesman said a vote against Quarta would not be in the interests of investors at a time “when the business needs stability until such time as a new chief executive is appointed”.

Some analysts have argued that the advertising giant should look to break up its assets, with speculation mounting over the future of data and market research business Kantar.