Syrian refugee children have been making clothes for British shoppers, it has been claimed.
An undercover BBC Panorama investigation of factories in Turkey reportedly found children working on clothes for Marks and Spencer and online retailer Asos.
The investigation also claimed that refugees were found to be working illegally on Zara and Mango jeans.
In response to the claims the brands involved said they carefully monitored their supply chains and do not tolerate the exploitation of refugees or children.
The investigation reportedly found seven Syrians working in one of Marks and Spencer's main factories and said they often earned little more than one pound per hour - well below the minimum wage.
The report alleged that they were employed through a middleman who paid them in cash on the street.
The British retailer said its inspections have not found a single Syrian refugee working in its supply chain in Turkey.
One of the refugees told the programme that they were poorly treated in the factory: "If anything happens to a Syrian, they will throw him away like a piece of cloth."
They also reported that the youngest worker was 15-years-old and he was working more than 12 hours a day, ironing clothes before they were shipped to the UK.
A spokesperson for Marks and Spencer said the programme's findings were "extremely serious" and "unacceptable to M&S".
They added that the retailer is offering permanent legal employment to any Syrians who were employed in the factory.
"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."
The investigation also alleged that in a back-street workshop in Istanbul where they discovered an Asos sample in the office, they found several Syrian children working.
Asos accepted that its clothes were made in the factory, but added that it was not an approved factory.
They continued that they had since inspected and found 11 Syrian adults and three Syrian children aged under-16 at work.
The spokesperson added that the children will be financially supported so they can return to school and the adults will be paid a wage until they have found legal work, adding: "We have implemented these remediation programmes despite the fact that this factory has nothing to do with Asos."
Reporter Darragh MacIntyre said he spoke to dozens of Syrian workers who 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."
The investigation also reported finding Syrian refugees working 12-hours days in a factory which distressed jeans for Mango and Zara.
It alleged the refugees were involved in spraying hazardous chemicals to bleach the jeans, but most of the workers did not even have a basic face mask.
Mango said the factory was working as a sub-contractor without its knowledge. They added that a subsequent inspection did not find any Syrian workers and found "good conditions except for some personal safety measures".
While a spokesperson for Zara's parent company, Inditex, says its factory inspections are a "highly effective way of monitoring and improving conditions". They added that they had already found signification non-compliance in an audit in June and had given the factory until December to make the required improvements.
The programme also reported finding Syrian adults working alongside Turkish children as young as 10 in another Istanbul factory.
They reported the owner said he had been working for Next and showed the undercover team a set of Next pyjamas he said the factory had helped produce.
Next said the pyjamas were made by another supplier, and those shown to the programme may have been a sample. It added that samples circulate widely and the presence of a sample in a factory does not mean it was made there.