U.S. Sen. Elizabeth Warren, D-Massachusetts, called on the Department of Labor this week to phase out a practice that allows employers to pay workers with disabilities less than the federal minimum wage -- a policy which she called "inherently discriminatory."

In a Monday letter to Labor Secretary Alexander Acosta, Warren and other senators requested information on the agency's oversight and enforcement of employers who use waivers to hire workers with disabilities and pay them below the federal minimum of $7.25 per hour.

Raising concerns about past abuses of the program, the lawmakers asked Acosta how DOL is working to prevent employers from mistreating and discriminating against workers with disabilities.

They further called on the secretary to responsibly end the practice.

"These waivers are inherently discriminatory and should be phased out in a responsible way," they wrote. "While the department continues to issue these waivers, however, we are concerned by past abuses of the program and hope to better understand the extent to which the Department is able to prevent employers' mistreatment of and discrimination against workers with disabilities."

Although the senators acknowledged that a section of the Fair Labor Standards Act lets DOL issue certificates allowing employers to pay workers with disabilities -- including those "impaired by age, physical or mental deficiency" -- a "special minimum wage," they cautioned that such waivers could lead to discrimination and unjustifiably low expectations.

The lawmakers, for example, pointed to reports suggesting that sub-minimum wage workers at Goodwill Industries sites have earned as low as 22 centers per hour, while those at a "vocational school" in Rhode Island "were forced to work long hours for little or no wages."

A USA Today report on the use of the waivers in New York, meanwhile, found such workers earned just 16 cents per hour at organizations that were paying tens of thousands of dollars in bonuses to their executives, they noted.

DOL public data suggested that as of January employers held more than 1,700 waivers, which covered 150,000-plus workers eligible for the lower wage -- including several in Massachusetts -- Warren's office noted.

Contending that several advocates and experts in the disability community have raised concerns about the potential for abuse under the DOL waivers, the senators noted that the Advisory Committee on Increasing Competitive Integrated Employment for Individuals with Disabilities has recommended a "well-designed, multi-year phase-out" of the practice, which allows those with disabilities to enter competitive integrated employment.

The committee, they added, has further recommended ways in which DOL can strengthen enforcement of the waivers and ensure they are issued only when "necessary in order to prevent the curtailment of opportunities for employment."

The senators asked Acosta to provide information on the pay rates for individuals with disabilities by employers using the certificates, the annual number of applications for the waivers and the number of inspections it has conducted on employers with the waivers, among other things.

U.S. Sens. Bob Casey, D-Pennsylvania; Patty Murray, D-Washington; Chris Van Hollen, D-Maryland; Maggie Hassan, D-New Hampshire; Tammy Duckworth, D-Illinois; and Bernie Sanders, I-Vermont, joined Warren in sending the letter.