NEW YORK (CNN/Money) - Corporate chieftains and investors should prepare for more boardroom and management changes as mutual fund managers and private pension funds join the proxy vote battle formerly led by the public pension funds.

CalPERS, the California state fund, which has $111 billion in stock in its portfolio and provides coverage to 1.4 million state employees and retirees, has said in the last week that it would vote its shares against the re-election of such corporate titans as Citigroup Chairman Sandy Weill to his company's board and Warren Buffett to the board of Coca-Cola. It has also joined public pension fund managers in New York, Connecticut and Illinois who are voting against management positions on the proxy vote at grocer Safeway.

But Calpers, by far the nation's largest public pension fund, controls no more than 0.6 percent of the shares outstanding at those companies. Private institutional investors such as mutual funds, hedge funds and private pension funds control the majority of the shares at all three companies.

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The public funds, often run by state officials with both political and investment criteria in mind, have been vocal in their disapproval of company management. But the private fund managers have generally kept their mouths shut, even when they have agreed with shareholder activists.

"Large funds, they make their living by accessing information from management," said Brian Heil, CEO of ProxyMatters.com, online proxy research resource. "The quickest way to turn off that spigot is to come out publicly against management."

All this is changing. Starting in August, a new Securities and Exchange Commission regulation will require all fund proxy votes to be disclosed publicly. This will mean that more private fund managers will join shareholder activists among public funds, according to officials with those funds and experts in the field.

"If they vote with management of a company that is not performing, I think that will require some explaining, given that individual investors are more attuned to issues," said Bill Atwood, executive director of the Illinois Board of Investment, which manages about $10.4 billion in assets on behalf of 125,000 Illinois state employees and retirees.

But, Atwood said, the push for corporate governance reforms at companies that are performing well will still be difficult.

"If the company has had good returns, voting with management is easier to explain to investors," said Atwood.

Patrick McGurn, special counsel for Institutional Shareholder Services, a firm that advises both public and private funds on proxy issues, said that the increased attention to corporate governance issues by individual investors is forcing the private funds to take positions very similar to the public fund gadflies.

McGurn and others say that even without the regulations, some private funds have taken relatively strong activist stands on corporate governance issues.

"We know when (mutual fund provider) Vanguard disclosed their voting plan, their positions were up there with the Calpers of the world," said McGurn. "There's any number of funds, including a couple corporate pension funds, that would surprise you, given their lack of voice on these issues, how strong they've been on these issues."

McGurn said private pension plans will now follow written policies when deciding how to vote on proxy questions. In the past, even companies with such policies might have waived them to vote in favor of a management that had provided good returns. They'll be unlikely to do that now for fear of shareholder lawsuits. Now, McGurn said, the process of drafting public proxy voting policies with the knowledge that votes will soon be disclosed, will get private pension funds to take a much stronger activist stance than in the past.

"There is a new form of activism out there," he said. "The critical mass of shareholder votes that could go against management has grown exponentially from last year to this year. There's going to be no routine voting items in the future."