Exporters in Delhi said last week that sales had increased by more than 60% as the fashion caught on. Swamped with orders, many had to subcontract embroidery to small workshops and middle-men.

Children who had been sold into bonded labour — virtual slavery — by their families in other parts of India were sent to work in Delhi to meet a surge in demand for traditional Indian zari embroidery and beading for the boho chic look.

Arun Bhutani, whose export company supplies Topshop and other leading high street names, said he had had so many orders from Britain and the United States for embroidery that he subcontracted out much of the work and had no idea whether child labour was used.

“We don’t allow child labour in the factory, but there’s no guarantee that it was not used. It’s not possible for everybody to check everything,” he admitted.

The campaigners say this is what the fashion industry must do to eradicate child labour. They are calling on designers and high street chains to guarantee that children have not made any part of their clothes, bags and shoes.

A spokesman for Topshop said the company allowed neither child labour nor subcontracting. “Topshop has not detected any child labour in our manufacturing process nor has it been made aware of this occurring by local stakeholders,” he said, adding that the company supported any independent initiative to protect the welfare of children.

The international fashion industry’s dirty secret is hidden in hundreds of cramped, dusty workshops on the outskirts of Delhi where an estimated 100,000 children work up to 14 hours a day.

The largely Muslim slums in Delhi’s Selampur and Kalakar suburbs could not be more remote from the glamour of the catwalk. In narrow lanes with open sewers, there are hundreds of one-room workshops, in each of which up to 15 children are forced to work long days for less than 3p an hour.

The workshops are filled with children from some of India’s poorest states, including Bihar and Jharkhand. In one of 10 workshops visited by The Sunday Times sat Fayaz, who appeared to be no older than eight, and Darinder, who claimed to be 15, but looked 12.

The two boys had been taken out of school in Bihar and brought to Delhi by their families. They were sitting back-to-back on a thin, rough carpet, each stitching tiny, glistening beads into patterns on pink chiffons stretched taut on wooden frames.

They and 11 other children worked from 9am to 9pm, with an hour’s lunch break, they said. They have one day off a week. They sleep on the floor beneath the zari frames.

Fayaz has an angelic face but his expression is fearful. He checks the boss is not listening before saying he does not know how old he is. “I earn 300 rupees (£3.50) a week. I miss my friends,” he said. “I went to school and I miss it.”

Darinder was handed over to the workshop two years ago after his mother died, leaving his father, a farmer, to raise him and his three brothers and sisters. “I earn 2,000 rupees a month. One thousand rupees goes in my pocket, the other 1,000 is sent home. I want to go home,” he said.

According to the anti-slavery Save Childhood campaign, thousands of children receive no pay for the first year until they are “trained”. A fashion trade expert who works for the group said: “There are thousands of workshops like these and 90% of them use child labour.”

Kailash Sathyarthi of the Global March Against Child Labour, said: “These children lose their opportunity to enjoy their childhood and they lose their constitutional right to an education. It is contemporary slavery.” Babs Mahil, a British-Indian designer who has made embroidered clothes for Tony and Cherie Blair and Richard Branson, urged the British fashion industry to back the campaign.

“If you’re subcontracting, you should know who you’re giving work to,” she said.

Last week the Indian government banned children from working as domestic servants or in hotels, cafes and roadside snack stands. The law on other sectors remains unclear.

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