Refugee children from Syria have been found making clothes for British fashion brands in Turkey.
Children as young as 15 were reportedly found working on clothes for Marks & Spencer and online retailer Asos, some working more than 12 hours a day.
Another factory that said it made clothes for Next employed Turkish children and Syrian refugees, while others were illegally working on Zara and Mango jeans, the BBC's Panorama programme found.
Investigators found seven Syrians working in one of Marks & Spencer's main factories, with refugees earning about £1 an hour - far below the Turkish minimum wage - and paid in cash on the street by a middleman.
The youngest worker, a 15-year-old, worked 12 hours a day ironing clothes before shipment to the UK.
Another refugee explained how poorly workers were treated, saying: "If anything happens to a Syrian, they will throw him away like a piece of cloth."
In another Istanbul workshop, where several Syrian children were at work, an Asos sample was found in the office.
More refugees were discovered at another factory distressing jeans for Mango and Zara, spraying hazardous bleaching chemicals without even the most basic of face masks.
And in one workhouse, Panorama found Syrian adults at work alongside Turkish children as young as 10. The owner said he had been working for Next and showed reporters a set of Next pyjamas he said the factory had helped produce.
According to Panorama, many clothes are made in Turkey due to its proximity to Europe and because it is used to dealing with last-minute orders, allowing retailers to rush new designs into shops quicker than if they were made elsewhere.
Most refugees did not have work permits and many were working illegally in the garment industry, the programme found.
Reporter Darragh MacIntyre said Syrian workers felt they were being exploited.
He said: "They speak of pitiful wages and terrible working conditions. They know they are being exploited but they know they can do nothing about it."
All of the brands told Panorama they carefully monitor their supply chains in Turkey and do not tolerate the exploitation of refugees or children.
Marks & Spencer said its inspections have not found a single Syrian refugee working in its supply chain in Turkey but called the findings "extremely serious" and "unacceptable", promising permanent legal employment to any Syrians who were employed in the factory.
It said: "Ethical trading is fundamental to M&S. All of our suppliers are contractually required to comply with our Global Sourcing Principles, which cover what we expect and require of them and their treatment of workers.
"We do not tolerate such breaches of these principles and we will do all we can to ensure that this does not happen again."
Asos accepted its clothes were made in the workshop Panorama discovered but said it was not an approved factory and promised to financially support the workers found there.
Mango said a subcontracted factory had been used without its knowledge, while Zara's parent company Inditex said factory inspections were a "highly effective way of monitoring and improving conditions".
Next said pyjamas were made by another supplier and those seen by Panorama may have been a sample.
But Danielle McMullan, from the Business and Human Rights Resource Centre, said brands need to understand they are responsible.
She said: "It's not enough to say we didn't know about this, it's not our fault. They have a responsibility to monitor and to understand where their clothes are being made and what condition they are being made in."
:: Panorama - Undercover: The Refugees Who Make Our Clothes, is on BBC One at 8.30pm on Monday.