Coalition of charities says women were systematically abused and faced retaliation if they reported the attacks, based on interviews with workers in Bangladesh, Cambodia and Indonesia.
Women who work in Asian factories making clothes for the global retail giant Walmart are at "daily risk" of slapping, sexual abuse and other harassment, rights groups said on Friday.
Based on interviews with about 250 workers in 60 Walmart supplier factories in Bangladesh, Cambodia and Indonesia, a coalition of charities said women were "systematically exposed to violence" and faced retaliation if they reported the attacks.
"This is a very urgent and serious issue," Anannya Bhattacharjee of the Asia Floor Wage Alliance, a group which represents garment workers, told the Thomson Reuters Foundation.
"All people see are the glittering, fast-moving and affordable fashion. No one has any idea about the deep-rooted violence against women that is propagated in the supply chains."
The charities have investigated the factories for more than six years as efforts mount to push Western brands into cleaning up the workplace and improving safety along their supply chains.
The charities said they found widespread sex harassment, verbal and physical abuse such as slapping and threats of retaliation when women refused sexual advances from bosses.
Walmart reviewing findings
US-based Walmart, with at least 11,000 stores in nearly 30 nations, said it was reviewing the findings of the report.
"The accounts by workers is concerning, and we take allegations like this seriously," a Walmart spokeswoman said.
"We have a robust supply chain monitoring program. Walmart's Standards for Suppliers lists our social and environmental expectations for our suppliers, specifically addressing the cultivation of a safe and healthy work environment," she added.
Tip of the iceberg
The alliance, which probed the abuses with four other groups, said in a 43-page report that the incidents represented the tip of the iceberg.
Stigma and the risk of retaliation means that many women keep quiet, according to the rights groups.
"The difficulty is women don't feel comfortable to report. How can they seek intervention from the unions when the union leaders are mostly men?" said Khun Tharo from the Phnom Penh-based charity Center for Alliance of Labour and Human Rights.
"There is no legal mechanism for them to file complaints."
The findings were shared in advance with the Thomson Reuters Foundation, a week before the International Labour Organization holds its first meeting on workplace violence and harassment.
Bhattacharjee said Walmart was investigated because of its global footprint and the charities hoped it could become a "trend setter" and put in place a system to stop such abuses.
A woman who worked in a factory in Bangladesh told the rights groups that her employer pressured her to resign after she threatened to report repeated abuses to police.
"He flirted with me, he would touch me on the shoulder or touch me on the head ... I thought if I showed no interest, he would stop. It didn't work," the woman said in the report.
Campaigners said the level of pressure and harassment faced by the workers in the study was approaching forced labour.
"Any time you have retaliation against workers, and coercion and control ... you are coming close to the line of forced labour," said Jennifer Rosenbaum with Global Labor Justice, a trans-national network of worker and migrant organisations.
Asia accounts for more than half of the $443 billion generated from global apparel exports in 2016, with Bangladesh, Vietnam, India, Hong Kong, Indonesia and Cambodia being the main players, according to the World Trade Organization.
Walmart, which is the largest private US employer, runs stores under its own name and owns companies from British supermarket chain ASDA to internet retailer Jet.com.