The noise was deafening and air in the factory in northern Gujarat was so thick with cotton dust it was like a snowstorm at night.
Women and girls, some no more than 10 or 11, fed machines with raw cotton picked from the nearby fields.
It is a process known as ginning - one end of a commercial supply chain that ends up as clothes and textiles in high street shops around the world. Globally, annual revenues from the industry are measured in the trillions of dollars.
Many household-name retailers concede they do not know exactly how the cotton they use is farmed and processed. Yet, for years, labour activists here have campaigned for their help.
"The workers' lives are terrible," said Jignesh Mevani, an activist who was our guide. "They are not paid the minimum wage. There are no safety precautions. There are many children."
Filming openly, children were easy to find in both the ginning factories and the cotton fields.
One was Kali Gamar, wearing a worn yellow dress, her arms covered in scratches from the cotton bushes. She thought she was 10, but wasn't sure.
Her expression was flat, her eyes dull as, almost robotically, she prised open cotton buds, one after the other. She was with her older sister, Ashi, who said she was 20.
"We came here four or five months ago from Rajastan," said Ashi. "Now we live here. The work is hard. We don't know where our parents are. They are working somewhere."
In one of the ginning factories, we found Versha and Pryanka. They were both 11 years old, far away from home and too shy or frightened to speak.
An adult worker, Gauri, explained they had been sent there through a labour agent by their parents.
"They came some months back," she said. "They don't get paid. The money must go straight to their parents."
Some estimates put the number of cotton child workers in India as high as half a million.
"A third of the workers may be children," says Sudhir Katiyar, who runs the campaigning organisation Prayas Centre for Labor Research and Action.
"Children are at every stage of the process, seeding cotton, picking it and ginning and beyond, too," he says.
Mr Katiyar refers to the cotton dust in the ginning factories as "the horror of the white cloud", as it can cause lung disease at an early age.
By law, he says, masks and safety equipment should be provided to all workers. Children should not work until they are 16.
There have been cases of them falling asleep through exhaustion and suffocating in the piles of raw cotton.
Workers are paid just over $2 (£1.30) a day, when they should be paid $7 for a 12-hour shift, Mr Katiyar insists.
From the ginning factories, the cotton is spun into thread from which textiles are made.
It is at this stage that the supply chain becomes a completely different world.
Mandhana Industries in Tarapur, two hours from Mumbai, is a supplier to many European brands.
Its plant is clean and modern, with an abundance of posters promoting good working practices. Staff wear protective masks and clothing.
"We have a social programme," says Mandhana president Ajay Prakash Bhatnagar. "We have doctors and health care. We take care of everything.
"I think this is why we get so many repeat orders again and again. Our customers are happy with the way we operate."
Usually companies like Mandhana track their supply chain to the spinning mills, while high street retailers track back to their immediate suppliers. Little or no scrutiny is made of the ginning factories and cotton fields.
None of the retailers we contacted agreed to be interviewed on this issue.
But Arcadia, owner of outlets such as Top Shop, BhS and Miss Selfridge, issued a statement saying: "We are committed to ensuring that the workers in our suppliers' supply chains are treated fairly.
"When customers buy our goods they must be sure that they have been produced under acceptable conditions."
Marks and Spencer said it did not break down what percentage of its raw cotton was traceable and guaranteed to be free of unacceptable labour practices.
"All our suppliers must adhere to our ethical standards as a condition of working with us," it said in a statement. "We do not tolerate abuse of these standards."
The company says, however, that it intends to be able to trace the source of its cotton by 2015.
Child labour has been reported in all the major cotton growing countries - China, India, Pakistan, Brazil, Uzbekistan and Turkey.
In response to the BBC findings in Gujarat, the British government said businesses were encouraged to remain vigilant about the work conditions for products they buy from overseas.