A proposed ballot measure that would allow California cities to change the retirement benefits of current government employees drew a quick, sharp response from CalPERS, the state's retirement system.

CalPERS said the ballot measure, which its backers said would ensure that the pension system is sustainable, runs counter to guarantees made by the state and federal constitutions, would force it to breach its fiduciary duty and would put into law measures that have been shot down by the courts.

"CalPERS is bound by fiduciary duty to deliver the promised pension benefits according to the U.S. and California constitutions, statutory law and case law," it said in a statement said. "CalPERS will continue to support and defend our members' vested rights, in accordance with the laws of the land and our obligations under the federal and state constitutions."