BOSTON/NEW YORK (Reuters) - T. Rowe Price Group Inc (>> T. Rowe Price Group Inc) plans to vote against key directors at companies with two classes of shares in the upcoming U.S. corporate annual meeting season, as the big asset manager steps up its push against companies that give extra voting clout to insiders.

Major U.S. corporations from Viacom Inc (>> Viacom, Inc.) to Facebook Inc (>> Facebook Inc) have classes of shares that give founders, CEOs and other insiders extra influence, creating a long-running debate over the balance of power between them and outside investors.

The change in policy has not been reported. Portfolio managers can vote counter to the policy, where warranted, the asset manager said.

Still, the change at one the largest publicly traded fund firms, with $763 billion in assets under management at the end of last year, sends a message to boardrooms and could boost efforts by activists to equalize voting rights at some established companies.

At Viacom, investors are calling for controlling shareholder Sumner Redstone to end a dual-class structure, saying it insulates leaders from investor pressure.

Meanwhile, Rupert Murdoch maintains his influence at Twenty-First Century Fox Inc (>> Twenty-First Century Fox Inc) and News Corp (>> News Corp) thanks to a similar structure, as do the founders of Google parent Alphabet Inc (>> Alphabet Inc). In all, more than 10 percent of companies in the Russell 3000 Index have unequal voting of shares, according to FactSet.

Donna Anderson, head of T. Rowe Price's corporate governance efforts, said the Baltimore-based company adopted the new policy last month after becoming alarmed by the rising number of initial public offerings with dual-class structures, mimicking the larger companies.

Proponents of new rules say outside investors should have an equal vote in corporate decisions, while backers of dual-class structures say they help ensure that founders and other trusted leaders can plan for the long term - and that outside investors understand the rules when buying shares.

There were no immediate signs that other big fund firms would make a similar change. A spokeswoman for Vanguard Group said it had no plans to change its voting policies to follow T. Rowe Price’s lead. A spokesman for BlackRock Inc (>> BlackRock, Inc.) declined to comment on the matter and Fidelity Investments did not immediately return a request for comment.

T. Rowe Price and other fund firms have backed efforts for change before, but the new policy specifically focuses on directors the company has backed in the past.

"To just endorse their re-election every year sends the wrong message," Anderson said in an interview this week.

Under the new policy, at dual-class U.S. companies with unequal voting rights, T. Rowe Price funds now will vote against lead independent directors, or independent chairs, and all nominating and corporate governance committee members.

While the new policy may not determine whether directors are re-elected, T. Rowe Price's size means the change will "make those directors take notice," said Laura Campos, director of shareholder activities for the Nathan Cummings Foundation, which has pressed News Corp to end its dual-class structure.

Since August, a half-dozen IPOs have included unequal voting rights, according to FactSet, including dating website operator Match Group Inc (>> Match Group Inc) and digital payments company Square Inc (>> Square Inc).

Shareholder resolutions calling for companies to give all shares an equal vote also have gained ground. At Ford Motor Co (>> Ford Motor Company) such a proposal received 36 percent support in 2015, up from 19 percent support in 2009, according to Manhattan Institute data.

On March 14 a vote on a similar advisory proposal will be held at Viacom. The proposal is not expected to pass as Redstone still controls a majority of the media company's voting stock.

(Reporting by Ross Kerber; Editing by Peter Henderson and Bill Rigby)

By Ross Kerber and Jessica Toonkel