Proxy adviser Institutional Shareholder Services on Monday joined calls for Research In Motion management to step away from leading the BlackBerry maker's board.
Shareholders will vote on the issue at RIM's annual meeting next month, after mutual fund Northwest & Ethical Investments called for the split between chairman and CEO roles.
Both jobs are shared by RIM co-founder Mike Lazaridis and by Jim Balsillie, who have struggled recently to defend the company's products against intense smartphone competition increasingly dominated by Apple and Google.
ISS, a unit of MSCI, said the Canadian company's board was not an adequate counterbalance to management and was unable to ensure independent oversight and accountability to shareholders.
There seems to be no compelling rationale for the company to resist handing over all of the duties of chair along with the title, ISS said in a report.
Lazaridis and Balsillie are also RIM's largest shareholders, with each holding a more than 5 percent stake.
Balsillie resigned as chairman in March 2007 and stepped down from the board in February 2009 following a stock options scandal. He returned, along with Lazaridis, as co-chairman in December 2010.
RIM has advised shareholders to vote against the motion at its annual meeting on July 12, saying independent board member John Richardson acts as de facto head of the board.
A RIM spokeswoman declined to comment on Monday beyond what the company said in a regulatory filing earlier this month.
Glass Lewis, a rival advisory firm, issued a report on Friday backing the split and also said shareholders should withhold a vote to return Richardson to the board.
ISS said it did not believe Richardson provides a sufficient counterbalance to the influence of Lazaridis and Balsillie.
RIM shares were down 1.75 percent at $28.07 on Nasdaq by early afternoon. They have fallen some 60 percent since a February peak around $70. The stock was down 1.8 percent in Toronto at C$27.71.
(Reporting by Alastair Sharp; editing by Rob Wilson)