The hum of sewing machines is a constant in parts of Guangzhou, a thriving port on the Pearl River in southern China.

It rattles through the open windows of factories from morning until late at night, as they finish the t-shirts, shorts, blouses, pants and swimwear that will be shipped to fill wardrobes in more than 150 countries.

This is the sound of Panyu, the neighbourhood known as the “Shein village”, a warren of factories that power the world’s largest fast fashion retailer.

“If there are 31 days in a month, I will work 31 days,” one worker told the BBC.

Most said they only have one day off a month.

The BBC spent several days here: we visited 10 factories, spoke to four owners and more than 20 workers. We also spent time at labour markets and textile suppliers.

We found that the beating heart of this empire is a workforce sitting behind sewing machines for around 75 hours a week in contravention of Chinese labour laws.

These hours are not unusual in Guangzhou, an industrial hub for rural workers in search of a higher income; or in China, which has long been the world’s unrivalled factory.