U.S. securities regulators adopted rules today to curb the risk of investor runs on institutional money market funds, mainly by requiring their value to float instead of maintaining a value of $1 per share.

A divided U.S. Securities and Exchange Commission approved the measure in a 3-2 vote, capping a years-long heated debate between regulators and the industry dating back to the financial crisis.

Republican SEC Commissioner Michael Piwowar and Democrat Kara Stein both voted against the measure.

The centerpiece of the SEC's reform will force "prime" money market funds used by large institutions to float their share price. Some critics have objected to this part of the rule, a major change from the current structure in which all money market funds maintain a stable $1 per share net asset value.

The rule will affect a variety of asset managers, from Blackrock Inc, Fidelity and Vanguard to Charles Schwab Corp, Pimco and Federated Investors Inc, as well as numerous companies and municipalities that rely on money funds.

The SEC's rule "creates a very strong reform package that significantly mitigates the risks of a run in money market funds and that will limit further contagion should a run occur," SEC Chair Mary Jo White said.

The reform was inspired by fallout from the Reserve Primary Fund in 2008. The fund's exposure to Lehman Brothers prompted panicked investors to yank their money. The fund "broke the buck," its net asset value falling below $1 per share.

The Federal Reserve was ultimately forced to backstop the industry until the chaos subsided.

The SEC hopes switching to a floating net asset value will prevent investors from getting spooked by the prospect of funds breaking the buck.

Another major provision of the rule will permit fund boards to lower so-called redemption "gates" or charge fees of up to 2 percent in stressed market conditions if a fund's weekly liquid assets fall below 30 percent of its total assets. Gates could only remain in place for 10 business days.

Both reforms are slated to go into effect two years after they are published in the Federal Register.

Piwowar, in his dissent, said he remains concerned with issues surrounding the floating net asset value. Stein said she fears the fees and gates may actually fuel firesales and runs.

Reform for the $2.6 trillion industry comes after a long battle between the SEC, the industry and federal banking regulators who sit on the Financial Stability Oversight Council (FSOC).

Opponents of the rules, including the U.S. Chamber of Commerce, warned that big changes in the structure of money market funds could cut off a major supply of short-term funding for corporations.

BlackRock said it “supports the SEC’s efforts to improve the resiliency of U.S. money market funds during times of stress" and added that it will work with clients "to discuss the wide array of cash solutions available to successfully adapt to these new reforms.”

Former SEC Chair Mary Schapiro initially pushed two potential plans for money funds, including either a floating net asset value or a capital buffer requirement. The final rule would not subject retail money market funds or government funds to a floating net asset value, because a run on those funds is considered less likely than on a fund for institutional investors.

Funds for tax-exempt municipal bonds would also not require a floating net asset value if they meet the definition of a "retail" fund.

The SEC said today that the U.S. Treasury Department and the Internal Revenue Service will unveil a plan permitting investors to use a simplified tax accounting method. Tax implications were a major sticking point for critics of the floating value. SEC Republican Commissioner Daniel Gallagher said Wednesday he understands the industry's concerns, and has launched a working group to track the rule's implementation.