The study by the Asia Floor Wage Alliance (AFWA) found workers stitching clothes for H&M in factories in Delhi and Phnom Penh faced problems such as low wages, fixed-term contracts, forced overtime and loss of job if pregnant.
The AFWA, a coalition of trade unions and labour rights groups, accused the Western high street retailer of failing on its commitments to clean up its supply chain.
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An official from H&M told the Thomson Reuters Foundation on Saturday that the fashion firm has been working actively to improve the lives of textile workers for many years.
"The report raises important issues and we are dedicated to contribute to positive long-term development for the people working in the textile industry in our sourcing markets," said Therese Sundberg from H&M's press and communications department.