The case concerns 220 Chinese workers found last December in conditions "analogous to slavery" at a BYD plant under construction in Camacari, in the northeastern state of Bahia.

Bahia's regional ministry for works (MPT) said in December it had found "degrading working conditions" at the site being built, expected to be BYD's largest electric car plant outside Asia.

Workers slept without mattresses and, in one case, 31 people had to share a bathroom, it said.

Laborers had "visible signs of skin damage" from working long hours under the sun.

The MPT said it also suspected "forced labor," with illegal clauses in workers' contracts, passports confiscated and the employer withholding as much as 70 percent of their salary. Workers were monitored by armed guards.

After the allegations were made public, BYD's Brazilian subsidiary said it had broken its contract with the Jinjiang contractor responsible for work on the site.

Jinjiang denied the slavery allegation.

The MPT is now seeking 257 million reais ($45.3 million) for "collective moral damages," as well as individual payments for each worker.

The civil suit against BYD, Jinjiang and Tonghe Intelligent Equipment (now Tecmonta) was filed after the companies refused to sign a "conduct adjustment agreement" proposed by Brazilian authorities, the MPT said.

On Thursday, BYD said in a statement it had collaborated with the MPT from the beginning, and "reaffirms its non-negotiable commitment to human and labor rights, guiding its activities by respecting Brazilian legislation and international labor protection standards."

Chinese foreign ministry spokeswoman Mao Ning told reporters Beijing "places great importance on protecting and safeguarding workers' legitimate rights and interests," and requires Chinese companies to "operate in compliance with laws and regulations."