"We are supporting the board (on re-election)," David Herro, Chief Investment Officer of international equity at U.S.-based Harris said in emailed comments to Reuters ahead of the company's annual meeting on Wednesday.

WPP is also asking investors to approve executive pay. Some investors say the company has not given them enough information about Sorrell's shock departure ahead of a vote on the company's remuneration report.

"(We support WPP on) board reelection. I want to make sure on the rest," Herro said in an email.

Harris is WPP's biggest fund investor with a 7.5 percent stake, according to Thomson Reuters data. Sorrell left WPP in April.

(Reporting by Simon Jessop, editing by Kirstin Ridley)