The shareholder activist group, RREV, raised concerns about the lack of succession planning at WPP ahead of the advertising giant's annual general meeting next week.
The RREV, run by National Association of Pension Funds and the US corporate governance organisation body ISS, said there was no hint who would succeed the 60-year-old Sorrell.
Sorrell has headed WPP for 20 years and built it from a supermarket basket manufacturer based in Kent, known as Wire & Plastic Products, to what is now the second biggest advertising group in the world with than 84,000 staff.
There has been no public discussion of when Sorrell will stand down and he is not expected to do so anytime soon, although he has a long way to go before he catches Ed Meyer, who is 76 years old and has been with Grey for 50 years.
As well as raising questions about who might succeed Sorrell at WPP, RREV also slated the advertising group for lax corporate governance.
According to a report in The Observer, the shareholder group raised concerns about the composition of the WPP board and the lack of independent non-executive directors.
RREV said that it did not consider non-executive director Bud Morten to be independent because of his long-time links with Sorrell nor does it consider non-executive chairman Phil Lader to be independent because he is also an adviser to Morgan Stanley, one of the WPP's investment banks.
Morten up for re-election and RREV is recommending investors abstain to show their dissatisfaction.
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