SAN FRANCISCO (MarketWatch) -- Google Inc. co-founders Sergey Brin and Larry Page, who still own nearly one-fifth of the Internet giant, disclosed Friday that they intend to significantly reduce their stake by selling roughly $5.5 billion worth of stock over five years.

The sales are significant, because they would effectively eliminate Brin and Page's control of their company by cutting their collective voting power below 50%. Still, the 48% voting power that the co-founders' would retain following the sales nonetheless constitutes a formidable position of influence.

In a regulatory filing with the Securities & Exchange Commission, Brin and Page, who started Google GOOG, +0.01% as graduate students at Stanford University, disclosed that under a five-year "diversification plan" adopted in November, they'll be selling 5 million shares each.

Brin and Page currently own roughly 57.7 million shares of common stock in Google, or about 18% of its outstanding capital stock. Their diversification plan is intended "to allow Larry and Sergey to sell a portion of their Google stock over time as part of their respective long-term strategies for individual asset diversification and liquidity," according to the regulatory filing.

At Google's closing stock price of $550.01 on Friday, the co-founders' sale of stock would result in proceeds of roughly $2.75 billion each.

Brin and Page have maintained significant ownership stakes in Google, while also relying on a dual-class stock structure at the company that currently grants them about 59% of the voting power of the company's outstanding capital stock.

After their planned five years of stock sales, their voting power would be reduced to roughly 48%, according to the regulatory filing.

Google shares dipped $6.47 to $545.25 in after-hours trading.

The search giant's stock price has been on a wild ride over the past two years, veering toward $250 in late 2008, before mounting a steady recovery throughout 2009.

Early last year, Google took the unusual step of resetting stock options for employees at lower prices, making it easier for them to cash in on their equity.