Investment guru John Bogle warns that his own creation is getting dangerously big.

The father of the index fund wrote in an opinion piece for The Wall Street Journal that it’s probably only a matter of time before they own half of all U.S. stocks.

The Vanguard founder and former CEO wrote that the "share of corporate ownership by index funds will continue to grow over the next decade" and warns that it is "only a matter of time until index mutual funds” cross the 50 percent mark.

“If that were to happen, the Big Three [Vanguard, State Street and BlackRock] might own 30 percent or more of the U.S. stock market — effective [voting] control,” he wrote.

“I do not believe that such concentration would serve the national interest,” he wrote.

Among the government-sponsored solutions Boggle proposes:

• More competition from new entrants to the index field.

• Force giant index funds to spin off their assets into a number of separate entities, each independently managed..

• Require index funds to hold just one company in any industry.

• Timely and full public disclosure by index funds of their voting policies and public documentation of each engagement with corporate managers.

• Require index funds to retain an independent supervisory board with full responsibility for all decisions regarding corporate governance.

• Enact federal legislation making it clear that directors of index funds and other large money managers have a fiduciary duty to vote solely in the interest of the funds’ shareholders.

“Even if present trends continue (sometimes they don’t), the enormous value of index funds should not be ignored,” he wrote.

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