Beyonce's much-hyped surprise collaboration with British clothing giant Topshop made headlines around the world when it launched in April this year. Topshop's website crashed as people rushed to purchase premium athleisure (prices start at $14 for sweatbands, and go up to $265 for a full length color block body.) An exclusive cover story with Elle emphasized how the clothing line was "a way to push a feel-good, woman-power ethos, to de-emphasize perfectionism, to value strength over beauty, and to inspire."

The backlash was inevitable. British tabloid newspaper the Sun on Sunday ran an expose describing the Sri Lankan garment workers making the range as "sweatshop 'slaves' earning just 44p (64 cents) an hour making 'empowering' Beyoncé clobber." Reporters visited "poverty-stricken seamstresses" at the MAS Holdings factory in Sri Lanka, which produces the clothes.

One sewing machine operator said that she was unable to survive on her basic wage of 18,500 rupees a month ($126). The newspaper claimed on average seamstresses earn £4.30 a day ($6.23), although acknowledging that workers at the factory were still being paid above the legal minimum wage of 13,500 rupees a month.

"What is being described by the Sun it looks like a severe case of exploitation, bordering on slavery," said anti-exploitation campaigner Jakub Sobik from Anti-Slavery International. He highlighted particular concerns, including low pay and limits on the women's movement at night by locking them into their accommodation (the workers reside on-site and are subject to curfews.)

Sobik called for Topshop to introduce independent inspections of all their suppliers, and asked them to "look more proactively into their supply chains to limit the risks of exploitation and modern slavery tainting their products."

To find out more about the conditions of the workers making Beyonce's clothes, we spoke to Dr Kanchana Ruwanpura of the University of Edinburgh. Dr Ruwanpura is an expert on the Sri Lankan garment industry, and has visited MAS factories in Sri Lanka, though her visit took place several years ago.

Read more: The Hellish Commute of the Women Who Make Your Clothes

"MAS are essentially top of the range in terms of labour conditions in Sri Lanka," she says. "They're brilliant factories in terms of the build space and the attention they usually pay to the codes they work with. However, I would say that when it comes to wages and freedom of association, MAS don't do a very good job."

Ivy Park launched in April this year. Photo courtesy of PR

Disturbingly, MAS workers are not allowed to unionize, despite the obvious benefits unions bring. "Factories across Sri Lanka, including MAS, are almost all guilty of not upholding the right of workers to unionize. That's one thing they completely fail on." Dr Ruwanpura highlights low wages as another concern. "MAS is not paying its workers a living wage."

Alongside tourism, the clothing industry in Sri Lanka is a huge economic driver. It directly employs an estimated 300,000 people, with an additional 600,000 indirectly employed. While Sri Lanka is generally considered to have better working conditions than other clothing-producing countries such as Bangladesh or Pakistan, campaigners say a living wage in the country should be 48,608 rupees ($332), over three times the legal minimum.

I ask Dr Ruwanpura about the life of an average Sri Lankan garment worker. "As none of the factories pay a living wage, salaries (with overtime included) are generally around the 17,000 to 20,000 rupee mark ($116 to $136/month.) The factories work on a shift system, apart from in busy periods when workers may be asked to work overtime to help make big orders."

But for all that Ivy Park's promotional materials talk of empowerment, how autonomous are the women sewing your mid-rise mesh-panelled leggings? "The garment industry has made Sri Lankan women financially independent, but I wouldn't say it's made them autonomous. When you're not being paid a living wage—well, it's economically challenging. I've been to some of these worker's houses. It's hard for them."