Google workers on Thursday expanded their rebellion against company practices, sending letters to three firms they say provide contract workers to Google, asking them to end mandatory arbitration for those workers.

The move follows a victory by Googlers who pushed the Mountain View technology giant to end forced arbitration for its own workers in March. Mandatory arbitration refers to the practice of requiring workers, as a condition of employment, to give up their right to sue in court over disputes and instead resolve them in negotiations that critics of the practice argue typically favor employers.

“This practice denies workers the right to hold their employer accountable for violations of civil liberties, scuttling any claims of illegal action,” the group Googlers for Ending Forced Arbitration said in their letters to contractor firms Adecco, Bon Appétit Management Company and CDI Corp.

“Worse, forced arbitration has a chilling effect, discouraging any reports of illegal action in the first place.”

The New York Times reported this week that contract and temporary workers at Google now outnumber full-time employees, with 121,000 temps and contractors globally and 102,000 full-time workers.

Google did not immediately respond to a request for comment. The three contractor firms did not immediately respond to questions, or confirm they currently impose arbitration agreements on workers, or say how many workers they provide to Google, and in which locations. Bon Appétit Management Company said in January it had used arbitration agreements for several years.

Palo Alto-based Bon Appétit Management Company operates company cafes for its clients. Adecco is a Swiss staffing company supplying workers in a variety of industries. CDI, headquartered in Philadelphia, is a staffing and outsourcing firm.

The Google workers’ group wants the three companies to “stop forcing (their) workers into arbitration for all disputes”  — including disagreements related to discrimination, harassment, wage theft and wrongful termination — and instead make arbitration optional, according to the letters.

The group’s letter campaign comes as Google workers increasingly push back against their employer. A protest against the company’s response to sexual harassment allegations, and payment of a $90 million golden parachute to Android creator Andy Rubin — who has denied claims that he forced a colleague into sex — led 20,000 employees to walk out of Google offices world-wide in November. Employees angry about Google’s work for the Pentagon prodded the company to back away from its work on the military’s “Project Maven” artificial-intelligence drone-warfare project. In August, more than 1,000 Googlers reportedly signed a letter opposing the company’s plan to launch a censored search engine in China.